Method and apparatus for controlling movement of cutting blade and workpiece

ABSTRACT

A method of controlling a relative movement of a cutting blade and a workpiece which are moved relative to each other by a movement device in an operation with a machine tool. The method includes a step of bringing the cutting blade and an object into contact with each other, by moving at least one of the cutting blade and the object toward each other by the movement device, and a step of controlling the relative movement on the basis of a relative position of the cutting blade and the object which is detected by the movement device upon the contact of the cutting blade and the object with each other. The method further includes a checking step of checking if a contact detecting device for detecting the contact of the cutting blade and the object is in a normal condition in which the contact detecting device is capable of detecting the contact when the cutting blade and the object are brought into contact with each other; and a contact determining step of determining that the cutting blade and the object have been brought into contact with each other, in accordance with an output provided by the contact detecting device.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of controlling aposition of a cutting blade in an operation with a machine tool, and anapparatus suitable for carrying out the method. It is noted that thepresent invention is applicable to any kind of cutting blades. Thecutting blade may be constituted by the entirety of a cutting tool, forexample, where the cutting tool consists of a solid tool which isprovided by a single piece, or may be constituted by a portion of acutting tool, for example, where the cutting tool includes a main bodyand a cutting tip or insert. In the latter case, the cutting blade isconstituted by the cutting insert, which is removably attached to themain body.

[0003] 2. Discussion of Related Art

[0004] In an operation with a machine tool, a cutting blade and aworkpiece are moved relative to each other, whereby the workpiece ismachined or cut by the cutting blade, so as to form a final orintermediate product having desired configuration and dimension. In theoperation, a relative position of the cutting blade and the workpiecehas to be appropriately controlled, for surely obtaining the desiredconfiguration and dimensions. For example, in an automatic machine toolsuch as a numerically controlled machine tool in which a movement devicefor moving at least one of the cutting tool and the workpiece iscontrolled in accordance with a predetermined program, the relativeposition of the cutting blade and the workpiece has to be accuratelydetected. Namely, it is necessary to accurately obtain a distance, asviewed in each of directions parallel to controlled axes of the machinedtool, between a predetermined portion of the workpiece (e.g., areference point in the workpiece which point serves as an origin ofcoordinates for a cutting program), and a cutting point of the cuttingblade when the cutting blade and the workpiece are positioned relativeto each other in a predetermined position (e.g., a “machine homeposition” which is a known position within a machining space of themachine tool). To this end, prior to an actual cutting operation, thereis conventionally required a step in which a touch probe (i.e., adetecting prove of a touch sensor) or a cutting blade is brought intocontact with an object (e.g., a master workpiece, and a referenceportion of the machine tool), and the relative position of the touchprobe (or the cutting blade) and the object upon contact of the touchprobe (or the cutting blade) with the object is read out from a positiondetecting device. In the cutting operation, the relative position of thecutting blade and the workpiece is controlled by controlling themovement device on the basis of the read-out contact position whichserves as a reference position.

[0005] The present invention is applied to an apparatus and a method ofdetecting the relative position of a cutting blade and a workpiece, bybringing the cutting blade into contact with the object, without using atouch sensor. For example, where an automatic lathe such as a NC(numerically controlled) lathe is used as the machine tool for cuttingan outer circumferential surface of the workpiece, a cutting point ofthe cutting blade is brought into contact with a surface of the object,by moving the cutting blade and the workpiece relative to each other ina radial direction of the workpiece. When the cutting point of thecutting blade is brought into contact with the surface of the object,the position of the cutting point and that of the contacted surface ofthe object is coincident with each other as viewed in the radialdirection. Therefore, if a relative position of the contacted surfaceand the workpiece is known, it is possible to accurately form theworkpiece into a product having desired configuration and dimension, bycontrolling the relative movement of the cutting blade and the workpieceon the basis of the contact position in which the cutting point of thecutting blade is brought into contact with the contacted surface of theobject.

[0006] Also where a milling machine or a machining center is used as themachine tool for cutting a workpiece with a rotary cutting tool, theworkpiece can be formed into a product having desired configuration anddimension in substantially the same manner as described above. However,in a cutting operation with a machining center, it is common that therotary cutting tool and the workpiece are both moved so that therelative movement required for the cutting operation is obtained bycombination of the movements of the cutting tool and the workpiece,although there is a case where only the rotary cutting blade is movedwhile the workpiece is held stationary. Where the rotary cutting tooland the workpiece are both moved, the relative movement is controlled bycontrolling a workpiece movement device for moving a workpiece holdingmember (e.g. a work table) which holds the workpiece, and also a toolmovement device for moving a tool holding member (e.g., a headstock)which rotatably holds a spindle into which the cutting tool is received.When the rotary cutting tool is brought into contact with an object soas to detect the relative position as the contact position, the rotarycutting tool and the object are moved toward each other with or withoutthe rotary cutting tool being rotated. Where the rotary cutting tool isbrought into contact with contacted object with the rotary cutting toolbeing rotated, it is possible to determine, as the contact position, therelative position in which the object is brought into contact with oneof cutting points of respective cutting blades of the cutting tool whichone has a lager radial distance from the axis of the cutting tool thanthe other cutting points. This is advantageous, particularly, in a casewhere a difference among the radial distances from the respectivecutting points to the tool axis has been increased, for example, due towear of the cutting points as a result of a long service of the cuttingtool.

[0007] Where the touch sensor is used for obtaining the above-describedcontact position, a required cost for the apparatus is increased due toexpensiveness of the touch sensor itself. Further, the use of the touchsensor is likely to cause a reduction in accuracy of positioning of thecutting blade unless a positional relationship between the touch sensorand the cutting blade is accurately known. These problems could beresolved by using the cutting blade in place of the touch sensor.However, the use of the cutting blade provides a risk of damaging thecutting blade, the object or holders holding the cutting blade and theobject, unless the contact of the cutting blade with the object issurely detected. For detecting the contact of the cutting blade with theobject, there is conventionally used an electric circuit including thecutting blade, the object and a power source which are arranged inseries. When the cutting blade and the object are separated from eachother, the electric circuit is open without an electric current flowingtherethrough. When the cutting blade is in contact with the object, theelectric circuit is closed whereby an electric current flowstherethrough. In this arrangement, it is possible to momentarily detectthe contact of the cutting blade with the object, and accordingly detectthe contact position, by detecting a state of the power source with adetector, namely, by detecting the electric current flowing from thepower source with a current detector. However, in the event of a failureof the power source or the detector, or a trouble with disconnection oflead wires of the electric circuit, the contact of the cutting bladewith the object would not be detected, whereby the cutting blade and theobject are further forced to each other, causing the above-describedrisk of damaging the cutting blade, the object or members holding thecutting blade and the object.

[0008] Even without the above-described failure or disconnectiontrouble, the use of the cutting blade for the contact with the objectwould suffer from various problems. For example, the contact of thecutting blade with the object is likely to cause “chipping” of thecutting blade, or otherwise damage or undesirably cut the object. Thatis, the cutting blade is likely to chip where the contact is madewithout rotation of the cutting blade or the object, while the object islikely to be damaged or undesirably cut where the contact is made withrotation of the cutting blade or the object. If a member having anextremely high degree of hardness is used as the object in the interestof avoiding undesirable cut of the object, the cutting blade would beworn or chipped more easily. Another problem is caused where theentirety of the cutting blade or at least the cutting edge of thecutting blade is provided by a material having a high degree of electricresistance or a material having substantially no electricalconductivity. That is, where the cutting blade is made of a ceramicmaterial, or where the cutting edge is made of a diamond sintered bodyor CBN (cubic boron nitrides) sintered body, its is extremely difficultor impossible to detect the contact of the cutting blade with theobject.

[0009] Further, the conventional technique for detecting the contact ofthe cutting blade with the object suffer from some other drawbacks, forexample, where the cutting blade attached to a holder of the machinetool is a wrong cutting blade which is not a cutting blade designated ina cutting operation program. More specifically, in a case where thecutting blade is provided by a replaceable cutting insert of a cuttingtool used for a lathe cutting operation, if the cutting blade is a wrongcutting blade (i.e., a wrong cutting insert), the wrong cutting blade islikely to be brought into contact with an unexpected portion of theworkpiece in the lathe cutting operation, whereby the cutting blade orthe workpiece could be damaged, or the workpiece could not be formedinto a product having desired configuration and dimension. In a casewhere the cutting blade is provided by a rotary cutting tool used for amilling operation, the position of the axis of the rotary cutting toolrelative to the workpiece is controlled during their relative movementin a direction perpendicular to the axis of the rotary cutting tool,i.e., in X- or Y-axis direction. Thus, in such a milling operation, ifthe wrong rotary cutting tool has a diameter different from that of acorrect rotary cutting tool, the workpiece is likely to be cut by thewrong rotary cutting tool with a radial depth of cut which is smaller orlarger than a desired value, thereby making it impossible to form theworkpiece into a product having desired configuration and dimension, andalso even causing a risk of damaging the rotary cutting and theworkpiece. It is needless to say that the same problems would beencountered where the wrong rotary cutting tool has an axial lengthdifferent from that of a correct rotary cutting tool, since theworkpiece is likely to be cut by the wrong rotary cutting tool with anaxial depth of cut which is different from that a desired value.

SUMMARY OF THE INVENTION

[0010] It is therefore an object of the present invention to improve thereliability of a detection of contact of a cutting blade and an object,in techniques for detecting a contact position in which the cuttingblade and the object have been brought into contact with each other, andthen controlling a relative position of the cutting blade and aworkpiece that is to be cut by the cutting blade, on the basis of thedetected contact position. In other words, the object of this inventionis to eliminate or minimize the above-described problems or drawbacksencountered in the conventional techniques.

[0011] The above object may be achieved according to any one of thefollowing modes of the present invention, each of which is numbered likethe appended claims and depends from the other mode or modes, whereappropriate, to indicate and clarify possible combinations of elementsor technical features. It is to be understood that the present inventionis not limited to the technical features or any combinations thereofwhich will be described for illustrative purpose only. It is to befurther understood that a plurality of elements or features included inany one of the following modes of the invention are not necessarilyprovided all together, and that the invention may be embodied withoutsome of the elements or features described with respect to the samemode.

[0012] (1) A method of controlling a relative movement of a cuttingblade and a work piece which are moved relative to each other by amovement device in an operation with a machine tool, the methodcomprising a step of bringing the cutting blade and an object intocontact with each other, by moving at least one of the cutting blade andthe object toward each other by the movement device, and a step ofcontrolling the relative movement on the basis of a relative position ofthe cutting blade and the object which is detected by the movementdevice upon the contact of the cutting blade and the object with eachother, the method further comprising:

[0013] a checking step of checking if a contact detecting device fordetecting the contact of the cutting blade and the object is in a normalcondition in which the contact detecting device is capable of detectingthe contact when the cutting blade and the object are brought intocontact with each other; and

[0014] a contact determining step of determining that the cutting bladeand the object have been brought into contact with each other, inaccordance with an output provided by the contact detecting device.

[0015] In the present method, at least one of the cutting blade and theobject is moved toward each other for achieving their mutual contact,after or while it is checked if the contact detecting device is held inthe normal condition in which the contact detecting device is capable ofdetecting the contact of the cutting blade and the object when they areactually brought into contact with each other. This arrangement iseffective to prevent failure to detect the actual contact, making itpossible to avoid a dangerous situation in which the above-described atleast one of the cutting blade and the object is further moved towardeach other even after the actual contact, causing a risk of damaging thecutting blade, the object or holders holding the cutting blade and theobject. The movement device is controlled to immediately stop themovement toward each other, in response to the detection of the contact.An experiment conducted by the present inventors revealed that a timerequired for stopping the movement after the detection of the contactcan be reduced to be considerably small, and that the cutting blade orthe object is not damaged by a minimized inertial motion or overrunningof the cutting blade and/or the object. It is noted that the term“cutting blade” may be interpreted to be constituted by the entirety ofa cutting tool, for example, where the cutting tool consists of a solidtool which is commonly made of a single material, or may be interpretedto be constituted by a portion of a cutting tool, for example, where thecutting tool includes a main body and a cutting tip or insert which iscommonly made of a material different from that of the main body. Ineither of the former and latter cases, the cutting blade includes atleast a cutting edge and an adjacent portion of the cutting tool whichportion is adjacent to the cutting edge.

[0016] (2) A method of controlling a relative movement of a cuttingblade and a workpiece which are moved relative to each other by amovement device in an operation with a machine tool, the methodcomprising a step of bringing the cutting blade and an object intocontact with each other, by moving at least one of the cutting blade andthe object toward each other by the movement device, and a step ofcontrolling the relative movement on the basis of a relative position ofthe cutting blade and the object which is detected by the movementdevice upon the contact of the cutting blade and the object with eachother, the method further comprising:

[0017] a first-circuit preparing step of preparing a first circuit whichincludes the object and the cutting blade arranged in series to eachother and which is connected to a power source, the first circuit beingclosed when the cutting blade and the object are in contact with eachother, while being open when the cutting blade and the object are not incontact with each other;

[0018] a second-circuit preparing step of preparing a second circuitwhich includes a resistance and which is connected in parallel with thefirst circuit, the second circuit being closed irrespective of whetherthe cutting blade and the object are in contact with each other or not;

[0019] a checking step of detecting a state of the power source by apower source detector when the first circuit is open, and checking ifeach of the power source and the power source detector is in a normalcondition; and

[0020] a contact-position determining step of determining, as a contactposition in which the cutting blade and the object are brought intocontact with each other, the relative position of the cutting blade andthe object when the state of the power source detected by the powersource detector indicates transition from an open state in which thefirst circuit is open, to a closed state in which the first circuit isclosed.

[0021] In the method according to this mode (2), when the cutting bladeand the object are not in contact with each other, an electric currentflows through the second circuit but does not flow through the firstcircuit since the first circuit is held open. On the other hand, whenthe cutting blade and the object are in contact with each other, thefirst circuit is closed so that the electric current flows through thefist circuit as well as through the second circuit. In this instance, anamount of the flowing electric current is increased since the first andsecond circuits are connected in parallel with respect to the electricpower. In view of this, for preventing the electric power or thedetector from being damaged by the flow of an excessively increasedamount of the electric current, it is preferable to employ, as the powersource, a power source having a current limiter which restricts the flowof the electric current to a certain amount, or alternatively provide aresistance having a certain resistance value in the first circuit. Theresistance to be provided in the first circuit may be constituted by asuitable device such as a fixed or variable resistor, or a main bodyportion of the machine tool, which is connected to the first circuit sothat a resistance inherently contained in the main body portion of themachine tool acts on the first circuit. In any one of these cases, it ispreferable that the resistance of the second circuit has a value notsmaller than that of the resistance of the first circuit, or morepreferable that the former has a value much larger than that of thelatter, so that the open state in which the first circuit is open can beeasily distinguishable from the closed state in which the first circuitis closed, on the basis of change of the amount of the flowing electriccurrent.

[0022] However, it should be noted that the value of the resistance ofthe second circuit has to be sufficiently small such that the flowingelectric current can be easily detectable by the detector even where theelectric current flows only through the second circuit, for enabling thedetector to surely detect a failure state in which the electric currentcan not flow through the second circuit, for example, due to a failureof the power source or the detector, or due to a disconnection or damageof a conductive member such as a lead wire. That is, in thisarrangement, the detector does not fail to detect such a failure statein which the contact of the cutting blade with the object is notdetectable. The relative movement of the cutting blade and the objecttoward each other under the failure state is advantageously prevented,thereby avoiding a risk of damage of the cutting blade or the object,which could be caused if the relative movement is erroneously continuedeven after the contact of the cutting blade with the object.

[0023] (3) A method according to mode (2), wherein the checking step isimplemented to detect a value of an electric current flowing from thepower source when the first circuit is open, and determine that each ofthe power source detector and the power source is in the normalcondition if the detected value of the electric current is not smallerthan a predetermined first value which is larger than zero and is notlarger than a predetermined second value which is larger than thepredetermined first value.

[0024] The detection of the state of the electric power may be achievedin various manners. For example, where an electric voltage betweenoutput terminals of the power source is momentarily reduced at themoment of an abrupt increase of the amount of the flowing electriccurrent upon closing of the first circuit, it is possible to detect thecontact of the cutting blade with the object, by detecting the reductionof the electric voltage. However, the contact of the cutting blade withthe object can be easier and more reliably detected by detecting anabrupt increase of the electric current supplied from the power sourceon the basis of the output of an electric current detector, rather thandetecting the reduction of the electric voltage.

[0025] (4) A method according to mode (3), wherein the contact-positiondetermining step is implemented to determine, as the contact position,the relative position of the cutting blade and the object when the valueof the electric current flowing from the power source exceeds apredetermined third value which is larger than the predetermined secondvalue.

[0026] The predetermined third amount is preferably larger than theamount of the electric current flowing through the first circuit when acutting point of the cutting blade and the a surface of the workpiececonnected to each other via a cutting fluid. Where the cutting blade andthe object are brought into proximity to each other with at least one ofthem being wet with the cutting fluid, the first circuit is almostclosed by the cutting fluid even before the contact of them, leading toan increase in the amount of the flowing electric current. Therefore, itis preferable that the predetermined third amount is sufficiently largesuch that such an increase in the amount of the electric current due tothe cutting fluid does not provide an erroneous determination that thecutting blade and the object are in contact with each other.

[0027] (5) A method of controlling a relative movement of a cuttingblade and a workpiece which are moved relative to each other by amovement device in an operation with a machine tool, the methodcomprising a step of bringing the cutting blade and an object intocontact with each other, by moving at least one of the cutting blade andthe object toward each other by the movement device, and a step ofcontrolling the relative movement on the basis of a relative position ofthe cutting blade and the object which is detected by the movementdevice upon the contact of the cutting blade and the object with eachother, the method further comprising:

[0028] a resistance-circuit preparing step of preparing a resistancecircuit in which the object and the cutting blade are connectable toeach other via a first resistance which has a predetermined firstresistance value;

[0029] a checking step of checking if a value of an electric resistanceacting between the object and the cutting blade is substantially equalto the predetermined first resistance value of the first resistance whenthe cutting blade and the object are not in contact with each other; and

[0030] a contact determining step of determining that the cutting bladeand the object have been brought into contact with each other when thevalue of the electric resistance acting between the object and thecutting blade is reduced to be smaller than a predetermined secondresistance value which is smaller than the first resistance value.

[0031] (6) A method according to any one of modes (1)-(5), wherein theobject consists of the workpiece which is fixed to the machine tool.

[0032] (7) A method according to any one of modes (1)-(5), wherein theobject consists of a master workpiece which is fixed to the machinetool.

[0033] The master workpiece may have a dimension substantially identicalto a desired dimension of a product which is to be formed from theworkpiece, or may have a dimension different from the desired dimensionof the product. In the latter case, it is possible to use the singlemaster workpiece as a common master workpiece, for operations in whichvarious kinds of products having different desired dimensions are formedfrom respective workpieces.

[0034] (8) A method according to any one of modes (1)-(5), wherein theobject consists of a reference portion of the machine tool.

[0035] (9) A method according to any one of modes (1)-(8), wherein thecutting blade consists of a cutting insert attached to a main body of acutting tool, the main body including a shank portion and cooperatingwith the cutting insert to constitute the cutting tool.

[0036] (10) A method according to any one of modes (1)-(8), wherein thecutting blade consists of at least a part of a rotary cutting tool whichis attached to a tool spindle of the machine tool.

[0037] (11) A method according to mode (10), wherein the cutting bladeand the object are brought into contact with each other while the rotarycutting tool is being rotated.

[0038] (12). A method according to any one of modes (2)-(4), wherein theresistance of the second circuit is connected between twomutually-insulated members, the method further comprising a coveringstep of covering a surface of at least one of the two mutually-insulatedmembers, with an insulating layer made of an electrically insulatingmaterial, for preventing the state of the power source from erroneouslyindicating the transition from the open state to the closed state whenthe two mutually-insulated members are shorted to each other by asubstance sticking to the mutually-insulated members while the cuttingblade and the object are not in contact with each other.

[0039] This arrangement is effective to prevent an erroneousdetermination of the transition from the open state to the closed statewhen the two-mutually insulated members are shorted to each other by thesticking substance such as a cutting fluid and cutting chips, namely,when the two members are connected through a by-passing passage which isformed of the sticking substance and which is positioned in parallelwith the resistance of the second circuit. It is noted that thetechnique, defined in this mode (12), of covering the surface of amember or members with the insulating layer may be applied to each ofthe methods defined in the above-described modes (5)-(11).

[0040] (13) A method according to any one of modes (1)-(12), furthercomprising a breakage determining step of determining that there is apossibility that the cutting blade has been damaged, if the contact ofthe cutting blade and the object is not detected while the relativeposition detected by the movement device satisfies a positionalcondition required for the contact of the cutting blade and the object.

[0041] In the method defined in this mode (13), it is determined thatthe cutting blade has been possibly damaged if the contact of thecutting blade and the object is not detected while the relative positiondetected by the movement device satisfies the positional relationshiprequired for achieving the contact of the cutting blade and the object.That is, this determination is made, for example, in a case where thecontact is not detected by the contact detecting device which is keptactivated while the cutting blade and the workpiece as the object havebeen apparently moved relative to each other to a relative position inwhich they should be in contact with each other in a process of theirrelative movement toward each other in an initiation of a cuttingoperation, or in a case where the contact is not detected by the contactdetecting device which is still kept activated for detecting contactwhile the workpiece is being cut by the cutting blade during a cuttingoperation. An operator, when this determination is made, can take anecessary procedure after checking the condition of the cutting blade,for example, in response to an activation of an alarm light or an alarmbuzzer informing the operator that the cutting blade has been possiblydamaged. In the method of this mode (13), the possible damage of thecutting blade is easily detected. It is preferable that the relativemovement of the cutting blade and the workpiece is automatically stoppedimmediately after the determination of the possible damage of thecutting blade has been made.

[0042] (14) A method according to any one of modes (1)-(13), furthercomprising a touch-probe detecting step of detecting a contact of theobject and a touch probe which is provided to be unmovable relative tothe cutting blade.

[0043] The use of the touch probe makes it possible to detect a positionwhich is impossible or difficult to be detected with the use of thecutting blade. For example, in an operation with a lathe, it is possibleto detect the position of an axis of the workpiece or the masterworkpiece which is fixed to a chuck of the lathe, by bringing aspherical contact end of the touch probe in contact with two portions ofan outer circumferential surface of the workpiece or master workpiecewhich portions are diametrically opposed to each other. Detecting such aposition as the axis is impossible or difficult by using the cuttingblade. After the position of the axis of the workpiece or masterworkpiece has been detected, the relative movement of the cutting bladeand the workpiece can be controlled on the basis of the detectedposition of the axis, if a position of the cutting blade relative to thetouch probe is known. It is preferable but not essential that the touchprobe is checked before the touch probe is brought into contact with theworkpiece or master workpiece in substantially the same manner as in theabove-described checking step. Where the step of checking the toughprobe is implemented, the implementation of the above-described checkingstep is not essential.

[0044] (15) A method according to any one of modes (1)-(14), wherein thecontact determining step includes an actual-contact determining step ofdetermining that the cutting blade and the object are actually broughtinto contact with each other when a predetermined first condition issatisfied, and wherein the predetermined first condition is differentfrom a predetermined second condition which is required to determinethat the cutting blade and the object are spaced apart from each otherand are shorted to each other by a cutting fluid, so that an actualcontact of the cutting blade and the object is distinguishable from ashort of the cutting blade and the object by the cutting fluid.

[0045] In the method defined in this mode (15), the predetermined firstand second conditions may include respective first and secondoutput-value-related requirements that a value of the output provided bythe contact detecting device is held in respective first and secondpredetermined ranges which are different from each other. For example,the value of the output may consist of a value of flowing electriccurrent. In this case, a lower limit of the first predetermined range isadapted to be larger than that of the second predetermined range, sincethe value of the flowing electric current is larger when the cuttingblade and the object are actually brought into contact with each other,than when they are shorted to each other by the cutting fluid whichinherently has a certain degree of resistance.

[0046] (16) A method of controlling a relative movement of a cuttingblade and a workpiece which are moved relative to each other by amovement device in an operation with a machine tool, the methodcomprising a step of bringing the cutting blade and an object intocontact with each other, by moving at least one of the cutting blade andthe object toward each other by the movement device, and a step ofcontrolling the relative movement on the basis of a relative position ofthe cutting blade and the object which is detected by the movementdevice upon the contact of the cutting blade and the object with eachother,

[0047] wherein the object consists of the workpiece which is fixed tothe machine tool, the method further comprising:

[0048] a dimension measuring step of measuring a dimension of theworkpiece on the basis of the relative position of the cutting blade andthe workpiece as the object upon the contact of the cutting blade andthe workpiece as the object with each other, so that the relativemovement is controllable on the basis of the measured dimension of theworkpiece.

[0049] There is known a sizing or measuring device which is attached toa machine tool so as to measure a dimension of a workpiece withoutremoving the workpiece from a workpiece holder holding the workpiece.Data representative of the measured dimension are utilized in a cuttingoperation so that the relative movement of the cutting blade and theworkpiece is controlled on the basis of the data. This controllingmethod with the measuring device is effective, particularly, for anoperation in which a high degree of dimensional accuracy is required.However, such a measuring device is commonly cable of measuring arelatively narrow range of dimension of the workpiece. On the otherhand, the method of this mode (16) makes it possible to accuratelymeasure a relatively wide range of dimension of the workpiece, owing tothe arrangement in which the cutting blade is used as a contact probebrought into contact with the workpiece, a portion of the movementdevice for moving at least one of the cutting blade and the workpiecerelative to each other is used to move the cutting blade serving as thecontact probe, and a portion of the movement device for detecting therelative position of the cutting blade and the workpiece is used as ascale for determining the dimension. The present method is accordinglyuseful to, particularly, a case where it is required to machine aworkpiece having a plurality of portions having respective targetdimensions which are considerably different from each other, or machinea plurality of workpieces having respective target dimensions which areconsiderably different from each other, with high precision. It is notedthat the method of this mode (16) can be carried out together with anyone or any combinations of the methods defined in the above-describedmodes (1)-(15).

[0050] (17) A method according to any one of modes (1)-(16), wherein thecutting blade is held by a blade holding member, and wherein the cuttingblade and the object are brought into contact with each other while aconductive layer having an electrical conductivity is provided in atleast one of a space between the cutting blade and the blade holdingmember, and a space between the cutting blade and the object.

[0051] The method defined in each of the above-described modes (1)-(16)can be carried out together with any one or any combinations of methodsdefined in modes (34)-(50) which are described below. It is noted thatthe term “blade holding member” may be interpreted to mean a main bodyof a cutting tool, for example, where the cutting tool includes the mainbody and the cutting blade in the form of a cutting insert which is heldby the main body, or may be interpreted to mean a cutting-tool holdingmember such as a tool turret or tool holder of a machine tool, forexample, where the cutting blade is provided by the entirety of acutting tool such as a solid tool.

[0052] (18) An apparatus for controlling a relative movement of acutting blade and a workpiece which are moved relative to each other bya movement device in an operation with a machine tool, the apparatuscontrolling the relative movement on the basis of a relative position ofthe cutting blade and an object which is detected by the movement devicewhen the cutting blade and the object are brought into contact with eachother as a result of a relative movement of the cutting blade and theobject which is made by the movement device, the apparatus comprising:

[0053] a checking device which checks if a contact detecting device fordetecting contact of the cutting blade and the object is in a normalcondition in which the contact detecting device detects the contact whenthe cutting blade and the object are brought into contact with eachother; and

[0054] a contact determining device which determines that the cuttingblade and the object have been brought into contact with each other, inaccordance with an output provided by the contact detecting device.

[0055] The method defined in the above-described mode (1) can beadvantageously carried out by using the apparatus defined in this mode(18).

[0056] (19) An apparatus for controlling a relative movement of acutting blade and a workpiece which are moved relative to each other bya movement device in an operation with a machine tool, the apparatuscontrolling the relative movement on the basis of a relative position ofthe cutting blade and an object which is detected by the movement devicewhen the cutting blade and the object are brought into contact with eachother as a result of a relative movement of the cutting blade and theobject which is made by the movement device, the apparatus comprising:

[0057] a first circuit which includes the object and the cutting bladearranged in series to each other and which is connected to a powersource, the first circuit being closed when the cutting blade and theobject are in contact with each other, while being open when the cuttingblade and the object are not in contact with each other;

[0058] a second circuit which includes a resistance and which isconnected in parallel with the first circuit, the second circuit beingclosed irrespective of whether the cutting blade and the object are incontact with each other or not;

[0059] a power source detector which detects a state of the powersource; and

[0060] a control device which commands the movement device to move thecutting blade and the object relative to each other if each of the powersource and the power source detector is indicated normal by the powersource detector when the first circuit is open, the control devicecontrolling the movement device on the basis of the relative position ofthe cutting blade and the object which is detected by the movementdevice when the state of the power source detected by the power sourcedetector indicates transition from an open state in which the firstcircuit is open, to a closed state in which the first circuit is closed.

[0061] The method defined in the above-described mode (2) can beadvantageously carried out by using the apparatus defined in this mode(19).

[0062] (20) An apparatus according to mode (19), further comprising aninsulator which electrically insulates a main body portion of themachine tool from the cutting blade wherein the resistance is connectedbetween the main body portion, and at least one of the cutting blade anda member which has an electrical continuity with the cutting blade.

[0063] (21) An apparatus according to mode (19), further comprising aninsulator which electrically insulates a main body portion of themachine tool from a workpiece holding member which holds the workpiece,wherein the resistance is connected between the main body portion, andat least one of the workpiece holding member and a member which has anelectrical continuity with the workpiece holding member.

[0064] The term “main body portion of the machine tool” recited in themodes (20) and (20) may be interpreted to include not only a mainstructure of the machine tool but also all components of the machinetool which have an electrically continuity with the main structure ofthe machine tool. That is, all components of the machine tool, locatedon one of opposite sides of the insulator which one is closer to themain structure rather than to the cutting blade, are included in the“main body portion”.

[0065] (22) An apparatus according to any one of modes (19)-(21),wherein the resistance includes a resistive layer which is interposedbetween members each made of an electrically conductive material.

[0066] The second circuit can be easily prepared by interposing theresistive layer between the members such as the components of the mainbody portion, a cutting-blade holding member, a cutting-tool holdingmember and a workpiece holding member each of which is commonly made ofa steel, brass or other material having a high degree of electricconductivity. However, a fixing device, which is provided for fixing themembers positioned on respective opposite sides of the resistive layer,has to be adapted to firmly fix the members relative to the resistivelayer while preventing the members from having an electrical continuitywith each other. If the resistive layer has a function of bonding themembers to the layer itself, the provision of the fixing device is notnecessary.

[0067] (23) An apparatus according to any one of modes (19)-(21),wherein the resistance consists of a resistor including a resistive bodyand a pair of terminals which are respectively disposed in opposite endsof the resistive body.

[0068] It is relatively difficult to adjust the electric resistanceacting on the second circuit to accurately have a desired value, byusing the resistive layer defined in the mode (23), due to thearrangement in which the resistive layer is held in contact at widesurfaces thereof with the members positioned on the respective oppositesides of the resistive layer. The value of the electric resistanceprovided by the resistive layer is likely to be changed depending uponan area of each of the contact surfaces of the layer. However, theresistor defined in this mode (23) makes it easy to adapt the electricresistance acting on the second circuit to accurately have a desiredvalue.

[0069] (24) An apparatus according to any one of modes (19), (20) and(23), wherein the resistance is built in one of a cutting tool whichincludes at least the cutting blade, and a tool holding member whichholds the cutting tool.

[0070] In general, cutting tools can be classified into two types. Acutting tool of one of the two types is constituted by a main body and acutting blade in the form of a replaceable cutting insert which isremovably fixed to the main body. Another type of cutting tool, which iscommonly called a solid tool, is constituted by a single piece whoseentirety can be considered to correspond to a cutting blade. A cuttingtool constituted by a main body and a cutting blade in the form of aninsert or tip which is fixedly welded or brazed to the main body couldbe interpreted to correspond to the above-described one type. However,for the sake of explanation, such a cutting tool having a brazed tip isdefined as a kind of solid tool, namely, interpreted to correspond tothe above-described another type in the descriptions of thisspecification. Therefore, in the descriptions, the cutting toolincluding the main body and the cutting blade is interpreted to mean acutting tool including a main body and a replaceable cutting blade whichis removably attached to the main body by suitable clamping means andwhich cooperates with the main body to constitute the cutting tool.

[0071] (25) An apparatus according to mode (21), wherein the resistanceconsists of a resistor built in one of a workpiece holding member whichholds the workpiece, and a component of a main body portion of themachine tool.

[0072] (26) An apparatus according to any one of modes (19)-(24),further comprising:

[0073] an insulator which electrically insulates a main body portion ofthe machine tool, from one of the cutting blade and the workpiece;

[0074] a first conductive passage which is connected at one of oppositeends thereof to the one of the cutting blade and the workpiece, andwhich is connected at the other of the opposite ends to the powersource;

[0075] a second conductive passage which connects the power source tothe main body portion of the machine tool;

[0076] a current detector which detects an electric current flowingthrough the first circuit that includes the first and second conductivepassages; and

[0077] a third conductive passage which connects the one of the cuttingblade and the workpiece, to the main body portion of the machine toolvia the resistance, the third conductive passage being included in thesecond circuit,

[0078] wherein the third conductive passage is shorter than the firstconductive passage.

[0079] The length of the first conductive passage can be reduced bydisposing the power source in the vicinity of the cutting blade. Thereduction of the length of the first conductive passage makes itpossible to form the first conductive passage with a conductive memberhaving a reduced length, thereby reducing a risk of disconnection ordamage of the conductive member. However, in general, the firstconductive passage requires to have a certain degree of length, becauseof a difficulty of disposing the power source and the detector in thevicinity of the cutting blade, or because of necessity of disposing thepower source and the detector in positions distant from the cuttingblade in the interest of minimizing splashing of the cutting fluid andcutting chips over the power source and the detector which can be easilydamaged. That is, for these reasons, it is not easy to reduce the lengthof the first conductive passage. On the other hand, the third conductivepassage can be easily adapted to have a small length, since theresistance provided in the third conductive passage is not easilydamaged by the splashing of the cutting fluid and cutting chips. In theevent of damage or disconnection of the third conductive passage, thecontrol device determines that the electric power, the detector or thefirst conductive passage is not in a normal condition even if each ofthese components is actually in the normal condition, and accordinglyinhibit the movement device from carrying out the relative movement ofthe cutting blade and the object toward each other. In this sense, it ispreferable to minimize the length of the third conductive passage, forreducing a possibility of the damage or disconnection of the thirdconductive passage.

[0080] (27) An apparatus according to mode (26), wherein the resistanceconsists of a resistor including a resistive body and a pair ofterminals which are respectively disposed in opposite ends of theresistive body, and wherein the resistor and the third conductivepassage are built in one of a cutting tool which includes at least thecutting blade, and a tool holding member which holds the cutting tool.

[0081] In the apparatus of this mode (27) in which the third conductivepassage, as well as the resistor, is built in one of the cutting tooland the tool holding member, the third conductive passage is protectedby the one of the cutting tool and the tool holding member, therebyavoiding a risk of the damage or disconnection of the third conductivepassage. The mode (28) described blow provides substantially the sametechnical advantage.

[0082] (28) An apparatus according to mode (26), wherein the resistanceconsists of a resistor including a resistive body and a pair ofterminals which are respectively disposed in opposite ends of theresistive body, and wherein the resistor and the third conductivepassage are built in one of a workpiece holding member which holds theworkpiece, and a component of a main body portion of the machine tool.

[0083] (29) An apparatus according to any one of modes (19)-(28),wherein the resistance of the second circuit is connected between twomembers, and wherein at least one of the two members is covered, at atleast a portion of a surface thereof which portion is adjacent to asurface of the other of the two members, with an insulating layer whichis made of an electrically insulating material.

[0084] (30) An apparatus according to any one of modes (19)-(29),further comprising:

[0085] a touch probe which is provided to be unmovable relative to thecutting blade, and;

[0086] an on-off circuit which includes an object and the touch probearranged in series to each other and which is connected to a powersource, the on-off circuit being closed when the touch probe and theobject are in contact with each other, while being open when the touchprobe and the object are not in contact with each other.

[0087] At least one of the object and the power source which are recitedin this mode (30) may be provided by the object and/or the power sourcewhich are recited in the above-described modes, or alternately, may beprovided by another object and/or another power source. It is noted thatthe apparatus of this mode (30) may further include (a) a movementdevice which moves the touch probe and the cutting blade relative to theworkpiece, and/or which moves the workpiece relative to the touch probeand the cutting blade, (b) a third circuit which includes a resistanceand which is connected in parallel with said on-off circuit, said thirdcircuit being closed irrespective of whether said touch probe and saidobject are in contact with each other or not, (c) a power sourcedetector which detects a state of said power source; and (d) a controldevice which commands said movement device to move said touch probe andsaid object relative to each other if each of said power source and saidpower source detector is indicated normal by said power source detectorwhen said on-off circuit is open, said control device controlling saidmovement device on the basis of said relative position of said touchprobe and said object which is detected by said movement device whensaid state of said power source detected by said power source detectorindicates transition from an open state in which said on-off circuit isopen, to a closed state in which said on-off circuit is closed.

[0088] (31) An apparatus according to any one of modes (19)-(30),wherein the control device determines that the cutting blade and theobject are actually brought into contact with each other when apredetermined first condition is satisfied, and wherein thepredetermined first condition is different from a predetermined secondcondition which is required to determine that the cutting blade and theobject are spaced apart from each other and are shorted to each other bya cutting fluid, so that an actual contact of the cutting blade and theobject is distinguishable from a short of the cutting blade and theobject by the cutting fluid.

[0089] (32) An apparatus according to any one of modes (19)-(31),wherein the control device include a breakage determining portion whichdetermines that there is a possibility that the cutting blade has beendamaged, if the contact of the cutting blade and the object is notdetected while the relative position detected by the movement devicesatisfies a positional condition required for the contact of the cuttingblade and the object.

[0090] (33) An apparatus according to any one of modes (18)-(32),wherein the cutting blade is held by a blade holding member, and whereinthe cutting blade and the object are brought into contact with eachother while a conductive layer having an electrical conductivity isprovided in at least one of a space between the cutting blade and theblade holding member, and a space between the cutting blade and theobject.

[0091] The apparatus defined in each of the above-described modes(18)-(32) can be carried out together with any one or any combinationsof cutting blades defined in modes (51)-(57), a master workpiece definedin mode (58), and a conductive sheet defined in mode (59), which aredescribed below.

[0092] The method or apparatus defined in each of the above-describedmodes (1)-(33) can be carried out together with any one or anycombinations of method, cutting blade, master workpiece, or conductivesheet defined in each of the following modes (34)-(59).

[0093] (34) A method of detecting contact and separation of a cuttingblade held by a blade holding member, with and from an object, on thebasis of a change of a state of an electric circuit which is changeddepending upon whether the cutting blade is in contact with the objector is separated from the object, the method comprising:

[0094] a step of bringing the cutting blade and the object into contactwith each other, while a conductive layer having an electricalconductivity is provided in at least one of a space between the cuttingblade and the blade holding member, and a space between the cuttingblade and the object.

[0095] The conductive layer, which is interposed between the cuttingblade and the blade holding member and/or between the cutting blade andthe object, for example, may take the form of a conductive coatingbonded to a surface of the cutting blade, the blade holding member orthe object; a local conductive coating bonded to a local portion of thecutting blade such as a portion including a cutting edge and an adjacentpart adjacent to the cutting edge; or a conductive sheet.

[0096] With the provision of the conductive layer between the cuttingblade and the object, the cutting blade and the object are brought intocontact with each other, necessarily through the conductive layerinterposed therebetween. The conductive layer does not impede thedetection of the contact of the cutting blade and the object, andadvantageously eliminates a risk of chipping of the cutting blade and arisk of damage or undesirable cut of the object, which might be causedwhere the cutting blade and the object are contacted directly with eachother. It is considered that the “indirect” contact position in whichthe two members are contacted with each other through the conductivelayer is offset from a “direct” contact position in which the twomembers are contacted directly with each other, by an amountcorresponding to a thickness of the conductive layer. However, where thethickness of the conductive layer is small enough to satisfy a requiredmachining accuracy, the indirect contact position can be taken as thedirect contact position. Where the thickness of the conductive layer isnot small enough, a position offset from the indirect contact positionby the amount corresponding to the thickness of the conductive layer canbe taken as the direct position.

[0097] Where the conductive coating is bonded to one of the cuttingblade and the object which are provided by respective conductive memberseach having a high degree of electrical conductivity, the conductivecoating can be advantageously made of a soft or brittle material, oralternatively made of a material having a predetermined degree ofelectric resistance. In the former case, it is preferable that theconductive coating is constructed such that a portion of the conductivecoating which portion covers a cutting edge and its adjacent portion ofthe cutting blade is destroyed rapidly upon initiation of a cuttingoperation. In this case, the conductive coating formed on the cuttingblade is brought into contact with the workpiece shortly before theinitiation of cutting of the workpiece with the cutting blade, so thatthe contact of the conductive coating with the workpiece is detectedbefore cutting blade starts to cut the workpiece. However, it is notdesirable that a portion of the conductive coating covering cutting edgeor edges not currently serving to cut the workpiece is destroyed due tocontact of such a portion of the coating with cutting chips or othersubstances. For preventing such a destruction of the portion of thecoating which cover the currently unused cutting edge or edges, it ispreferable that a degree of strength of the conductive coating is heldin a predetermined range. In the latter case, i.e., where the conductivecoating is made of a material having a predetermined degree of electricresistance, it is possible to avoid flow of an excessively high amountof electric current through contact portions of the cutting blade andthe object at which the two members are brought into contact with eachother. In this sense, the conductive coating made of the resistivematerial can be considered as a kind of electrically resistive coating.

[0098] The conductive coating covering the cutting edge and its adjacentportion may be made of a material, which is selected among a pluralityof materials having respective electric resistance values different fromeach other, depending upon kind of the cutting blade. In thisarrangement, an amount of change in value of an electric current or anelectric resistance representative of the state of the electric circuitupon the contact of the cutting blade with the object varies dependingupon the kind of the cutting blade. It is accordingly possible toidentify the kind of the cutting blade on the basis of the amount ofchange in the value of the electric current flowing through the electriccircuit, or on the basis of the amount of change in the value of theelectric resistance acting on the electric circuit. For example, theamount of change in value of the electric current or resistance can bedetected by a suitable detector so that the detected amount of change iscompared with a predetermined amount of change corresponding to acorrect cutting blade, i.e., a cutting blade designated in a cuttingoperation program. If the detected amount of change is different fromthe predetermined amount of change, it is determined that the cuttingblade which has been brought into contact with the object is not thecorrect cutting blade, namely, it is determined that the cutting bladein question is a wrong cutting blade which has been attached to theblade holding member by mistake. Such an identification of the cuttingtool can be made by comparing the value itself of the electric currentor resistance in stead of the amount of change in value of the electriccurrent or resistance. That is, irrespective of whether the amount ofchange in the value or the value itself is checked, it is possible todetermine if the cutting blade attached to the blade holding member andbrought into contact with the object is a currently required cuttingblade or not, simply by detecting or measuring the value of the electriccurrent or resistance in the electric circuit including, for example,the object, the conductive layer, the cutting blade and the bladeholding member which are arranged in series. In this arrangement, it isimportant that the material forming the conductive layer has a suitableresistance value. In this sense, the conductive coating can beconsidered as a kind of electrically resistive coating.

[0099] Where at least a contact portion of one of the cutting blade andthe object at which portion the one of the two members is brought intocontact with the other is made of an electrically insulating material,the conductive layer is formed to cover at least the contact portion.This arrangement is effective to provide the electrically insulatingcontact portion with an electric conductivity, as well as a resistanceto chipping or damage of the cutting blade. For example, where thecutting blade is provided by a cutting insert made of a ceramic materialwhich has a high degree of electric resistance or does not have anelectric conductivity, such a ceramic cutting insert may be covered atits entirety with a conductive layer so that the ceramic cutting insertcan be used as a conductive cutting insert having a high degree ofelectric conductivity. Where a rake face (which cooperates with a flankface adjacent to the rake face, to define a cutting edge or point at anintersection of the rake and flank faces) of the cutting blade iscovered with a diamond sintered body or CBN (cubic boron nitrides)sintered body, such a sintered body and its adjacent portion may becovered with a conductive layer so that contact of a cutting point ofthe cutting blade with the object can be detected. In this arrangement,it is important that the material forming the conductive layer has asufficient degree of electric conductivity. However, the conductivelayer can be adapted to have a predetermined electric resistance valuein addition to the sufficient degree of electric conductivity, ifneeded.

[0100] Further, the conductive layer may be provided in the spacebetween the cutting blade and the blade holding member, in addition toor in place of the space between the cutting blade and the object. Forexample, where the conductive layer is interposed between the cuttingblade and the blade holding member both of which have a high degree ofelectrical conductivity, a contact of the cutting blade and the objectprovides a change in the state of the electric circuit. Such a change inthe state of the electric circuit upon the contact of the two members isinfluenced by the conductive layer. Where the electric circuit isconstructed such that an electric current flows through the conductivelayer, as described below in mode (36), a value of the electric currentflowing through the conductive layer varies depending upon a surfacearea of the conductive layer, particularly, if the conductive layer ismade of a material having a high degree of electric resistance.Therefore, it is possible to determine whether or not the cutting bladebrought into contact with the object is a correct cutting blade, namely,whether or not the cutting blade currently attached in the blade holdingmember is a currently required blade in accordance with a cuttingoperation program. Still further, the conductive layer may be formed ona contact surface of the cutting blade which is held in contact with theblade holding member such that a value of electric resistance of theconductive layer is different from that of a conductive layer that isformed on other cutting tool. This arrangement permits the amount ofchange in the state of the electric circuit upon the contact, to varyfrom cutting blade to cutting blade, thereby making it possible toidentify the cutting tool actually brought into contact with the objectand accordingly to determine whether or not the cutting blade currentlyattached in the blade holding member is a currently required blade inaccordance with a cutting operation program.

[0101] (35) A method according to mode (34), wherein the contact and theseparation are detected on the basis of transition from an open state inwhich an on-off circuit as the electric circuit is open, to a closedstate in which the on-off circuit is closed, and wherein the on-offcircuit includes at least the cutting blade, the object and a powersource which are arranged in series to each other, the on-off circuitbeing open when the cutting blade is separated from the object whilebeing closed when the cutting blade is in contact with the object.

[0102] The method of this mode (35) can be advantageously practiced,particularly, where the cutting blade constitutes the entirety of acutting tool, namely, where the cutting tool consists of a solid tool.

[0103] (36) A method according to mode (34), wherein the contact and theseparation are detected on the basis of transition from an open state inwhich an on-off circuit is open, to a closed state in which the on-offcircuit is closed, and wherein the on-off circuit includes at least theblade holding member, the cutting blade, the object and a power sourcewhich are arranged in series to each other, the on-off circuit beingopen when the cutting blade is separated from the object while beingclosed when the cutting blade is in contact with the object.

[0104] The method of this mode (36) can be advantageously practiced,particularly, where the cutting blade constitutes a portion of a cuttingtool, namely, where the cutting tool includes a main body to which thecutting blade is removably attached. That is, this method can beadvantageously, for example, where the cutting blade is provided by areplaceable cutting insert which is replaceably attached to a main bodyof a cutting tool designed for a turning (lathe), milling, drilling,reaming, boring or other cutting operation.

[0105] (37) A method according to any one of modes (34)-(36), whereinthe conductive layer consists of a conductive coating which covers asurface of the cutting blade.

[0106] The conductive layer may take the form of the conductive coatingcovering the surface of the cutting blade. This arrangement facilitatesthe interposition of the conductive layer between the cutting blade andthe blade holding member, or between the cutting blade and the object.For example, by covering all the surface of the cutting blade with theconductive coating, the conductive layer can be easily interposedbetween the cutting blade and the blade holding member and at the sametime between the cutting blade and the object. However, the conductivecoating does not have to cover necessarily all the surface of thecutting blade, but may cover only the cutting edge and its adjacentportion of the cutting blade, or alternatively, only at least a portionof a contact surface of the cutting blade at which surface the cuttingblade is held in contact with the blade holding member, irrespective ofwhether the cutting blade is of a lathe cutting tool or of a rotarycutting tool.

[0107] (38) A method according to any one of modes (35)-(37), whereinthe conductive layer consists of a conductive coating which covers acontact surface of the object which surface is in contact with thecutting blade.

[0108] This arrangement in which the conductive coating covers theobject in stead of the cutting blade also facilitates the interpositionof the conductive layer between the cutting blade and the object.

[0109] (39) A method according to mode (38), wherein the object consistsof a master workpiece which has a known dimension and which is held by aworkpiece holding device that is provided for holding a workpiece to becut by the cutting blade.

[0110] (40) A method according to mode (35) or (36), wherein theconductive layer consists of a conductive sheet which is positioned tobe interposed between the cutting blade and the object when the cuttingblade and the object are in contact with each other.

[0111] The use of the conductive sheet makes it possible to interposethe conductive layer between the cutting blade and the object, evenwhere neither the cutting blade nor the object is covered with theconductive layer or coating.

[0112] (41) A method according to any one of modes (34)-(36), whereinthe cutting blade is provided by at least a cutting edge of a rotarycutting tool which is to be rotated about an axis thereof for cutting aworkpiece, and an adjacent portion of the rotary cutting tool whichportion is adjacent to the cutting edge,

[0113] wherein the cutting edge and the adjacent portion is covered witha conductive coating as the conductive layer,

[0114] and wherein the rotary cutting tool is brought into contact withthe object while the rotary cutting tool is being rotated.

[0115] The cutting blade may constitute the entirety of the cuttingtool, or may cooperate with the main body to constitute the cuttingtool. In the later case, the cutting blade may take the form of areplaceable cutting blade which is removably attached to the main body.In either of these cases, the contact of the rotary cutting tool and theobject can be detected without necessity of bringing the cutting edge ofthe tool into direct contact with the object.

[0116] (42) A method according to mode (41), wherein the rotary cuttingtool is brought into contact with the object while the rotary cuttingtool is being rotated at a velocity substantially equal to that at whichthe rotary cutting tool is rotated in a cutting operation for cuttingthe workpiece.

[0117] The method of this mode (42) makes it possible to accuratelydetect the contact position in which the rotary cutting tool and theobject are brought into contact with each other, even where the cuttingtool has a plurality of cutting edges whose respective radial distancesfrom the rotary axis are different from each other. In other words, thismethod makes it possible to determine, as the contact position, therelative position in which the object is brought into contact with oneof the cutting edges which has a larger radial distance from the rotaryaxis than the other cutting edges. Further, since the velocity at whichthe cutting tool is rotated as it is brought into contact with theobject is adapted to substantially equal to the velocity at which thecutting tool is rotated in the cutting operation, the contact positionis detected under the same conditions (e.g., vibrations caused by therotation of the cutting tool) as in the cutting operation. Therefore,the control of the relative movement of the cutting blade and theworkpiece on the basis of the contact position which is obtained in thismethod provides a further improvement in the dimensional accuracy of theproduct.

[0118] (43) A method according to any one of modes (34)-(36), whereinthe conductive layer consists of a resistive coating which covers atleast a cutting edge of the cutting blade and an adjacent portion of thecutting blade which portion is adjacent to the cutting edge, the methodfurther comprising:

[0119] a cutting-blade identifying step of determining that the cuttingblade is a currently required cutting blade if the state of the electriccircuit satisfies a predetermined condition when the cutting blade is incontact with the object via the resistive coating, and determining thatthe cutting blade is not the currently required cutting blade if thestate of the electric circuit does not satisfy the predeterminedcondition when the cutting blade is in contact with the object via theresistive coating, the predetermined condition including at least one ofa current-value-related requirement that a value of electric currentflowing through the electric circuit upon contact of the cutting bladewith the object via the resistive coating is held in a predeterminedrange, and a resistance-value-related requirement that a value ofelectric resistance acting on the electric circuit upon the contact isheld in a predetermined range.

[0120] (44) A method according to any one of modes (34)-(36), whereinthe conductive layer consists of a resistive coating which covers atleast a contact surface of the cutting blade that is held in contactwith the blade holding member, the method further comprising:

[0121] a cutting-blade identifying step of determining that the cuttingblade is a currently required cutting blade if the state of the electriccircuit satisfies a predetermined condition when the cutting blade is incontact with the object via the resistive coating, and determining thatthe cutting blade is not the currently required cutting blade if thestate of the electric circuit does not satisfy the predeterminedcondition when the cutting blade is in contact with the object via theresistive coating, the predetermined condition including at least one ofa current-value-related requirement that a value of electric currentflowing through the electric circuit upon contact of the cutting bladewith the object via the resistive coating is held in a predeterminedrange, and a resistance-value-related requirement that a value ofelectric resistance acting on the electric circuit upon the contact isheld in a predetermined range.

[0122] (45) A method according to any one of modes (34)-(36), whereinthe cutting blade held by the blade holding member includes a cuttingedge, an adjacent portion adjacent to the cutting edge and a contactsurface held in contact with the blade holding member, and wherein atleast the cutting edge, the adjacent portion and the contact surface ofthe cutting blade are covered with a conductive coating as theconductive layer, the method further comprising:

[0123] a moving step of moving at least one of the cutting blade and theobject toward each other such that the cutting edge is brought intocontact with the object via the conductive coating;

[0124] a memorizing step of memorizing, as a contact position, arelative position of the cutting blade and the object upon contact ofthe cutting edge with the object via the conductive coating;

[0125] a movement-controlling step of controlling a relative movement ofthe cutting blade and a workpiece which is to be cut by the cuttingblade, on the basis of the contact position memorized in the memorizingstep; and

[0126] a cutting-blade identifying step of determining that the cuttingblade is not a currently required cutting blade if the state of theelectric circuit including the workpiece, the cutting blade and theblade holding member which are arranged in series to each other, doesnot satisfy a predetermined condition when the conductive coatingcovering the cutting edge is destroyed due to contact of the cuttingblade with the workpiece as a result of the relative movement of thecutting blade and the workpiece, wherein the predetermined conditionincludes at least one of a current-value-related requirement that avalue of electric current flowing through the electric circuit upondestruction of the conductive coating is held in a predetermined range,and a resistance-value-related requirement that a value of electricresistance acting on the electric circuit upon destruction of theconductive coating is held in a predetermined range, the predeterminedcondition being determined to be not satisfied if the above-described atleast one of the current-value-related requirement and theresistance-value-related requirement is not satisfied.

[0127] In the method of this mode (45) in which the relative position ofthe cutting blade and the object upon the contact of the two members viathe conductive coating is taken as the contact position, the contactposition can be detected without the object being undesirably cut by thecutting blade. Further, when the cutting blade is brought into directcontact with the workpiece as the result of the destruction of theconduct coating caused by the relative movement of the cutting blade andthe workpiece toward each other, at least one of the values of theelectric resistance and the electric current in the electric circuit isdetected, so that it is determined that the cutting blade in question isnot a currently required cutting blade if the detected value or valuesis not in the predetermined range or ranges.

[0128] (46) A method according to mode (45), wherein the object consistsof the workpiece which is to be cut by the cutting blade.

[0129] In the method of this mode (46) in which the workpiece is used asthe object, the detection of the contact position and the determinationas to whether the cutting blade is a current required cutting blade ornot can be made in an initial stage of the cutting operation.

[0130] (47) A method of identifying a plurality of cutting blades eachincluding a cutting edge and an adjacent portion which is adjacent tothe cutting edge, comprising:

[0131] a covering step of covering at least the cutting edge and theadjacent portion of each of the cutting blades with a resistive coatingmade of a material, which is selected among a plurality of materialshaving respective electric resistance values different from each other,depending upon kind of each of the cutting blades;

[0132] a blade setting step of setting one of the cutting blades in ablade holding member such that the one of the cutting blades is held bythe blade holding member, for thereby forming an electric circuitincluding the one cutting blade, the blade holding member and an objectwhich is contactable with and separable away from the cutting blade andwhich is arranged in series with the one cutting blade and the bladeholding member; and

[0133] a blade identifying step of identifying kind of the one cuttingblade which is currently held by the blade holding member, on the basisof a state of the electric circuit when the cutting edge of the cuttingblade is in contact with the object via the resistive coating.

[0134] (48) A method of identifying a plurality of cutting blades eachincluding a cutting edge and a contact surface which is held in contactwith a blade holding member, comprising:

[0135] a covering step of covering at least the contact surface of eachof the cutting blades with a resistive coating made of a material, whichis selected among a plurality of materials having respective electricresistance values different from each other, depending upon kind of eachof the cutting blades;

[0136] a blade setting step of setting one of the cutting blades in ablade holding member such that the one of the cutting blades is held atthe contact surface by the blade holding member, for thereby forming anelectric circuit including the one cutting blade, the blade holdingmember and an object which is contactable with and separable away fromthe cutting blade and which is arranged in series with the one cuttingblade and the blade holding member; and

[0137] a blade identifying step of identifying kind of the one cuttingblade which is currently held by the blade holding member, on the basisof a state of the electric circuit when the cutting edge of the cuttingblade is in contact with the object.

[0138] The resistive coating may be adapted to cover the entire surfaceof the cutting blade, so that the cutting blade can be applied to themethod defined in the above-described mode (47). However, it ispreferable that a portion of the cutting blade which portion is broughtinto contact with the object is not covered with the resistive coating,for facilitating identification of the cutting blade. This is because,as described below in the DETAILED DESCRIPTION OF THE PREFERREDEMBODIMENTS, an electric resistance acting on a portion between thecutting blade and the object is likely to be considerably larger thanthat acting on a portion between the cutting blade and the blade holdingmember, and this relatively large electric resistance acting on theformer portion makes the identification of the cutting blade difficult.

[0139] (49) A method according to mode (47) or (48), wherein the stateof the electric circuit is represented by at least one of a value ofelectric current flowing through the electric circuit, and a value ofelectric resistance acting on the electric circuit.

[0140] (50) A method according to mode (49), further comprising ablade-selection checking step of determining that the one cutting bladeis a currently required cutting blade if a predetermined condition issatisfied, and determining that the one cutting blade is not thecurrently required cutting blade if the predetermined condition is notsatisfied, wherein the predetermined condition includes at least one ofa current-value-related requirement that the value of the electriccurrent is held in a predetermined range, and a resistance-value-relatedrequirement that the value of the electric resistance is held in apredetermined range.

[0141] (51) A cutting blade which is removably held by a blade holdingmember, for thereby cutting a workpiece, the cutting blade being coveredat at least a portion of a surface thereof with a conductive coatinghaving an electrical conductivity.

[0142] (52) A cutting blade according to mode (51), wherein theconductive coating is made of a material whose electric resistance islarger than that of a material of the cutting blade.

[0143] (53) A cutting blade according to mode (51), wherein theconductive coating is made of a material whose electric resistance issmaller than that of a material of the cutting blade.

[0144] (54) A cutting blade according to any one of modes (51)-(53),being covered at all the surface with the conductive coating.

[0145] (55) A cutting blade according to any one of modes (51)-(54),consisting of a replaceable cutting insert which is replaceably attachedto a main body of a cutting tool, the main body including a shankportion and cooperating with the replaceable cutting insert toconstitute the cutting tool.

[0146] (56) A cutting blade according to any one of modes (51)-(55),consisting of a cutting edge of a rotary cutting tool, and an adjacentportion of the rotary cutting tool which portion is adjacent to thecutting edge, the rotary cutting tool being rotated about an axisthereof for achieving a cutting operation.

[0147] (57) A cutting blade according to mode (56), wherein the cuttingedge and the adjacent portion are provided by a replaceable cuttinginsert which is replaceably attached to a main body of the rotarycutting tool, the main body including a shank portion and cooperatingwith the replaceable cutting insert to constitute the rotary cuttingtool.

[0148] (58) A master workpiece which is to be held by a workpieceholding device serving for holding a workpiece to be cut by a cuttingblade, and which is to be brought into contact with the cutting blade,the master workpiece having a known dimension and covered at a surfacethereof with a conductive coating which has an electrical conductivity.

[0149] (59) A conductive sheet consisting of a sheet member which ismade a material having an electrical conductivity.

BRIEF DESCRIPTION OF THE DRAWINGS

[0150] The above and other objects, features, advantages and technicaland industrial significance of the present invention will be betterunderstood by reading the following detailed description of presentlypreferred embodiments of the invention, when considered in connectionwith the accompanying drawings, in which:

[0151]FIG. 1 is a front view of a NC lathe which is equipped with acutting-blade-position controlling apparatus constructed according to afirst embodiment of the invention;

[0152]FIG. 2 is a block diagram schematically showing theabove-described cutting-blade-position controlling apparatus;

[0153]FIG. 3 is a cross sectional view showing a part of theabove-described cutting-blade-position controlling apparatus;

[0154]FIG. 4 is a cross sectional view showing a another part of theabove-described cutting-blade-position controlling apparatus;

[0155]FIG. 5 is a block diagram schematically showing acutting-blade-position controlling apparatus constructed according to asecond embodiment of the invention;

[0156]FIG. 6 is a block diagram schematically showing acutting-blade-position controlling apparatus constructed according to athird embodiment of the invention;

[0157]FIG. 7 is a block diagram schematically showing acutting-blade-position controlling apparatus constructed according to afourth embodiment of the invention;

[0158]FIG. 8 is a block diagram schematically showing acutting-blade-position controlling apparatus constructed according to afifth embodiment of the invention;

[0159]FIG. 9 is a view showing a cutting tool which is used in acutting-blade-position controlling apparatus constructed according to asixth embodiment of the invention;

[0160]FIG. 10 is a block diagram schematically showing acutting-blade-position controlling apparatus constructed according to aseventh embodiment of the invention;

[0161]FIG. 11 is a block diagram schematically showing acutting-blade-position controlling apparatus constructed according to aneighth embodiment of the invention;

[0162]FIG. 12 is a block diagram schematically showing acutting-blade-position controlling apparatus constructed according to aninth embodiment of the invention;

[0163]FIG. 13 is a block diagram schematically showing acutting-blade-position controlling apparatus constructed according to atenth embodiment of the invention;

[0164]FIG. 14 is a view showing a cutting insert constructed accordingto the invention;

[0165]FIG. 15 is a view showing a cutting insert which is a modificationof the cutting insert of FIG. 14;

[0166]FIG. 16 is a view showing a cutting tool constructed according toan eleventh embodiment of the invention;

[0167]FIG. 17 is a view showing a rotary cutting tool constructedaccording to a twelfth embodiment of the invention;

[0168]FIG. 18 is a view showing a rotary cutting tool constructedaccording to a thirteenth embodiment of the invention;

[0169]FIG. 19 is a block diagram schematically showing acutting-blade-position controlling apparatus constructed according to afourteenth embodiment of the invention;

[0170]FIG. 20 is a view showing a cutting insert constructed accordingto a fifteenth embodiment of the invention;

[0171]FIG. 21 is a view showing a master workpiece constructed accordingto a sixteenth embodiment of the invention; and

[0172]FIG. 22 is a view showing a conductive sheet, together with acutting tool and a workpiece, which is constructed according to aseventeenth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0173] Referring first to FIGS. 1-4, there will be described acutting-blade-position controlling apparatus which is constructedaccording to a first embodiment of this invention. Thiscutting-blade-position controlling apparatus is built in a NC(numerically controlled) lathe which is principally constituted by amain structure 10. The main structure 10 includes a base 12, a column 14which extends upwardly from the base 12, and a bed 16 and a headstock 18which are attached to a vertically intermediate portion of the column14. The headstock 18 holds a main spindle 22 such that the main spindle22 is rotatable about its rotary axis and is unmovable in its axialdirection. A tailstock 24 is disposed on the bed 16 such that thetailstock 24 is movable toward and away from the headstock 18. Thetailstock 24 holds a tail center 26 such that tail center 26 is opposedto and coaxial with the main spindle 22. A workpiece holding member inthe form of a three-jaw universal chuck 28 is attached to the mainspindle 22. A workpiece 32, which is to be machined or cut in the NClathe, is held at its axial end portion by this chuck 28 so as to berotatable with the chuck 28, or alternately, the workpiece 32 may beheld at its axially opposite end portions by the chuck 28 and the tailcenter 26 so as to be rotatable with the chuck 28 and the tail center26. In the latter case, the chuck 28 may be replaced with a main center(not shown) which is attached, in place of the chuck 28, to the mainspindle 22, so that the workpiece 32 is held at its axially opposite endportions by the main center and the tail center 26.

[0174] The NC lathe includes a Z-axis guide 36 which is attached to anupper portion of the column 14 and which extends in a Z-axis directionthat is parallel to the rotary axis of the main spindle 22; and acarriage 38 which is held by the Z-axis guide 36 and which is movable inthe Z-axis direction. The NC lathe further includes a X-axis guide 42which is provided in the carriage 38 and which extends in a X-axisdirection that is perpendicular to the rotary axis of the main spindle22; and a cross slide 44 which is held by the X-axis guide 42 and whichis movable in the X-axis direction. A cutting-tool holding member in theform of a turret 46 is fixed to cross slide 44. A cutting tool in theform of a lathe cutting tool 48 is attached to the turret 46. Thecarriage 38 is moved by a Z-axis movement device 56 which includes aZ-axis motor 52 and a Z-axis feed screw 54, while the cross slide 44 ismoved by a X-axis movement device 66 which includes a X-axis motor 62and a X-axis feed screw 64, so that the turret 46 and the lathe cuttingtool 48, which is attached to the turret 46, are moved in the Z-axis andX-axis directions.

[0175] The Z-axis and X-axis motors 52, 62 consist of respective servomotors which are numerically controllable, and are provided withrespective rotary encoders 72, 74. These encoders 72, 74 are connectedto a control device 78 as shown in FIG. 2, so that the control device 78calculates a position of the turret 46 or the cutting tool 48 (which ismoved by the movement devices 56, 66) relative to the headstock 18 orthe chuck 28, on the basis of outputs provided by the encoders 72, 74.That is, the position of the turret 46 or the cutting tool 48 in theZ-axis and X-axis directions is controlled in a feed-back manner. Thecontrol device 78 is principally constituted by a computer 80, andincludes, in addition to the computer 80, a driving circuit for drivingmotors such as the Z-axis and X-axis motors 52, 62, and a convertingcircuit for converting various outputs into digital data which can beinputted to the computer 80. The computer 80 includes a ROM (read-onlymemory) 82, a RAM (random-access memory) 84, a PU (processing unit) 86and I/O port 88. A cutting operation program for cutting the workpiece32 is stored in the ROM 82.

[0176] A contact detecting circuit 94 for detecting a contact of thecutting tool 48 with a master workpiece 92 is provided to be connectedto the control device 78. The contact detecting circuit 94 includes afirst circuit 96, a second circuit 98 and an electric current detector100. The first circuit 96, in which the cutting tool 48, the masterworkpiece 92 and a DC power source 104 are arranged in series with eachother, is closed when the cutting tool 48 and the master workpiece 92are in contact with each other, and is open when the cutting tool 48 andthe master workpiece 92 are not in contact with each other. The masterworkpiece 92 has a diameter substantially identical to a desireddiameter of a product which is to be formed from the workpiece 32. Themaster workpiece 92 is attached to the chuck 28, to which the workpiece32 is to be attached in a cutting operation. The second circuit 98 isconnected, in parallel with the first circuit 96, to the DC power source104, and includes an electric resistance in the form of a fixed resistor106. The second circuit 98 is held closed irrespective of whether thecutting tool 48 and the master workpiece 92 are in contact with eachother or not. The current detector 100 is a kind of a detector fordetecting a state of the DC power source 104, and is adapted to detectan electric current flowing from the DC power source 104 toward thefirst and second circuits 96, 98.

[0177] As shown in FIG. 2, the cutting tool 48 includes a main body inthe form of a tool body 110, and a cutting blade in the form of areplaceable cutting insert 112 which is attached to the tool body 110 byclamping means (not shown). The fixed resistor 106 is disposed betweenthe cutting insert 112 and a main body portion of the NC lathe, namely,between the tool body 110 and the turret 46, as shown in FIG. 3. Thefixed resistor 106 includes an electric resistive body 114 and a pair ofterminals 116, 118 which are respectively disposed in opposite ends ofthe resistive body 114. The fixed resistor 106 and a compression coilspring 124, which is a kind of elastic member, are accommodated in anaccommodating cavity 122 formed in the turret 46.

[0178] The fixed resistor 106 and the coil spring 124 are received in agenerally cylindrical housing 128 which is made of an electricallyinsulating material, such that the resistor 106 is movable relative tothe cylindrical housing 128 in an axial direction of the cylindricalhousing 128. The cylindrical housing 128 is press-fitted in theaccommodating cavity 122 so that the resistor 106 and the coil spring124 are held in the cavity 122. In an example illustrated by FIG. 3, thecavity 122 has a bottom while the cylindrical housing 128 has openingsin its axially opposite ends. An inward flange 130 is provided in one ofthe axially opposite opening ends of the housing 128, and extendsradially inwardly from a cylindrical wall of the housing 128. Thehousing 128 is fitted in the cavity 122 such that the axial opening endin which the inward flange 130 is provided is positioned in an openingend portion of the cavity 122. In this arrangement, the coil spring 124is compressed and biases the resistor 106 toward the inward flange 130.A protrusion 132 is provided to protrude from a central portion of theterminal 116 of the resistor 106, over a distance which permits theprotrusion 132 to protrude outwardly of the inward flange 130 when theresistor 106 is brought into contact with the inward flange 130.

[0179] The tool body 110 and the cutting insert 112 of the cutting tool48 and also the turret 46 are made of respective conductive materialseach having a high degree of electrical conductivity, while the toolbody 110 and the turret 46 are insulated from each other by electricallyinsulting members. In the example illustrated by FIG. 3, the insultingmembers are provided by an insulating layer 138 formed in a contactsurface of the turret 46 that is held in contact with the tool body 110,and another insulating layer 142 formed in a surface of a contact plate144 which is disposed between the tool body 110 and a bolt 140 which isprovided to fix the tool body 110 to the turret 46. With the tool body110 being fixed to the turret 46, the tool body 110 is held brought incontact with the protrusion 132 of the resistor 106, and accordinglyforces the resistor 106 toward the bottom of the accommodating cavity122 against a biasing force of the coil spring 124. In this arrangement,the tool body 110, the resistor 106, the coil spring 124 and the turret46 are forced to each other. That is, with the cutting tool 48 beingfixed to the turret 46, the cutting tool 48 and the turret 46 areelectrically connected to each other via a series circuit which isconstituted by the resistor 106 and the coil spring 124. It is notedthat the insulating layer 138 may be formed in accordance with knownmethods such as a PVD (physical vapor deposition) method, a CVD(chemical vapor deposition) method and a method of spray-forming aceramic coating or film.

[0180] A connecting portion 150 is provided in the turret 46 to connectthe cutting tool 48 to the DC power source 104 while electricallyinsulating the cutting tool 48 from the turret 46. The connectingportion 150 includes a terminal 154 and an electrical-continuityestablishing portion 156. The terminal 154 is fixed to the turret 46 andwhich is insulated from the turret 46 by an electrically insulating body152. The electrical-continuity establishing portion 156 includes ahousing 160 which is made of an electrically insulating material, acontact member 162 which is held in contact with the tool body 110, andan elastic member in the form of a coil spring 164 which is interposedbetween the contact member 162 and the terminal 154. In thisarrangement, the tool body 110, the contact member 162, the coil spring164 and the terminal 154 are forced to each other by an elastic force ofthe coil spring 164, for thereby establishing an electrical continuitybetween the terminal 154 and the tool body 110 of the cutting tool 48. Alead wire 166 is provided to be connected at one of its opposite ends tothe terminal 154. This lead wire 166 passes through the current detector100, and is connected at the other of its opposite ends to one of twoterminals of the DC power source 104. Another lead wire 168 is providedto be connected at one of its opposite ends to the other of the twoterminals of the DC power source 104, and connected at the other of itsopposite ends to a component of the main body portion of the NC lathe,for example, a component of the main structure 10.

[0181] As is clear from the above descriptions, in the presentembodiment, the insulating layers 138, 142 constitute an insulator whichelectrically insulates the cutting insert 112 as a cutting blade fromthe main body portion of the NC lathe as a machine tool. The contactmember 162, the coil spring 164 and the lead wire 166 cooperate to eachother to constitute a first conductive passage which is connected at oneof its opposite ends to the cutting blade and which is connected at theother of its opposite ends to the power source. The lead wire 168constitutes a second conductive passage which connects the power sourceto the main body portion of the machine tool. The coil spring 124constitutes a third conductive passage which connects the cutting bladeto the main body of the machine tool via the fixed resistor 106 as anelectric resistance. The third conductive passage provided by the coilspring 124 advantageously has a length much smaller than that of thefirst conductive passage provided by the contact member 162, the coilspring 164 and the lead wire 166, and is advantageously built in thecutting-tool holding member in the form of the turret 46. Further, thecoil spring 124 providing the third conductive passage is advantageouslyforced, at its opposite ends, onto the fixed resistor 106 and the turret46 by its own elastic force. These arrangements are effective tominimizing a risk of deteriorating the electrical-continuity between thetool body 110 and the turret 46 which are electrically connected by thecoil spring 124, i.e., the third conductive passage.

[0182] In the NC lathe equipped with the contact detecting circuit 94which is constructed as described above, an operation for cutting theworkpiece 32 is carried out as follows:

[0183] The operation is initiated by attaching the cutting tool 48 andthe master workpiece 92 to the turret 46 and the chuck 28, respectively,so that the first and second circuits 96, 98 are formed. Describedspecifically, the cutting tool 48, the current detector 100, the DCpower source 104, the main body portion of the NC lathe and the masterworkpiece 92 are arranged in series in the first circuit 96, while thecutting tool 48, the current detector 100, the DC power source 104, themain body portion of the NC lathe and the fixed resistor 106 arearranged in series in the second circuit 98. An electric resistancedenoted by a sign R1 in FIG. 2 represents an electric resistance actingon the second circuit 98 Thus, the electric resistance R1 actuallyincludes not only the fixed resistor 106 but also an electric resistanceprovided by the turret 46 and other parts of the main body portion ofthe NC lathe. However, the electric resistance R1 may be interpreted torepresent the fixed resistor 106, since the electric resistance providedby the main body portion of the NC lathe has a considerably smallresistance value. It is preferable that the fixed resistor 106 has aresistance value which is determined depending upon the value of theelectric resistance provided by the main body portion of the NC latheand also a value of an electric resistance provided by a cutting fluidused in the cutting operation, such that the resistance value of theresistor 106 is not smaller than 50 Ω and is smaller than 500 Ω. In thepresent embodiment, the resistance value of the resistor 106 is set tobe 250 Ω. An electric resistance denoted by a sign R2 represents anelectric resistance acting on the first circuit 96. Thus, the electricresistance R2 may be interpreted to represent an electric resistanceprovided by the main spindle 22 and the other parts of the main bodyportion of the NC lathe. In the present embodiment, this electricresistance has a value not larger than 1 Ω. That is, the value of theelectric resistance R1 is much larger than that of the electricresistance R2, and their relationship can be expressed by the followinginequality:

R1>>R2

[0184] In an initial stage of the operation in which a cutting point ofthe cutting insert 112, i.e., a distal end or cutting point of thecutting tool 48 is in a position distant from the master workpiece 92,the first circuit 96 is held in open state in which the first circuit 96is open. In the conventional apparatus, an electric current is notsupplied from the power source to the electric circuit in such aninitial stage, as discussed above in the Discussion of Related Art. Onthe other hand, in this embodiment of the invention, even when thecutting point of the cutting tool 48 is separated from the masterworkpiece 92, a small amount of electric current flows through thesecond circuit 98 which is held closed irrespective of whether thecutting tool 48 is in contact with the master workpiece 92 or not. Thiselectric current is detected by the current detector 100, but thedetected value of the current is small due to the electric resistance R1having a large resistance value. However, the detected value isobviously larger than zero, and is not smaller than a predeterminedfirst value and not larger than a predetermined second value that islarger than the predetermined first value. The computer 80 of thecontrol device 78 operates to execute a program for reading the detectedvalue of the electric current, and then determines that the contactdetecting circuit 94 is in a normal condition if the read value of theelectric current is not smaller than the predetermined first value andnot larger than the predetermined second value. The normal condition isinterpreted to means a condition in which an electric voltage is appliedbetween the cutting point of the cutting tool 48 and the masterworkpiece 92, for enabling the circuit 94 to detect a contact of thecutting tool 48 and the master workpiece 92 when the contact of the twomembers 48, 92 is actually achieved. This step of checking if thecontact detecting circuit 94 is in the normal condition is referred toas a “checking step”. If it is not determined at the checking step thatthe contact detecting circuit 94 is in the normal condition, thecomputer 80 inhibits the movement devices 56, 66 from carrying outrelative movement of the cutting tool 48 and the master workpiece 92toward to each other. This arrangement is effective to prevent failureto detect the actual contact of the cutting tool 48 and the masterworkpiece 92, making it possible to avoid a dangerous situation in whichat least one of the cutting tool 48 and the master workpiece 92 isfurther moved toward each other even after their actual contact, withouta risk of damaging the cutting tool 48, the master workpiece 92 orholders holding the cutting tool 48 and the master workpiece 92.

[0185] If it is determined at the checking step that the contactdetecting circuit 94 is in the normal condition, the computer 80 of thecontrol device 78 operates to execute a program for controlling theZ-axis and X-axis motors 52, 62 of the respective Z-axis and X-axismovement devices 56, 66, such that the cutting tool 48 is moved towardthe master workpiece 92, so as to be brought into proximity to an outercircumferential surface of the master workpiece 92. The cutting tool 48is moved toward the master workpiece 92 at a high feed rate (rapid feedrate) while they are distant from each other. The cutting tool 48 isthen decelerated at a predetermined decelerating position which is nearto the master workpiece 92. Thus, the cutting tool 48 is brought intoproximity to the outer circumferential surface of the master workpiece92, with its approaching movement at a low feed rate (cutting feedrate). It is preferable this approaching movement of the cutting tool 48to the outer circumferential surface of the master workpiece 92 iscarried out by moving the cutting tool 48 only in the radial directionof the master workpiece 92, i.e., only in the X-axis direction.

[0186] Where a cutting fluid is used in the operation, there is apossibility that the cutting point of the cutting tool 48 is broughtinto connection with the master workpiece 92 via the cutting fluidbefore the cutting point of the cutting tool 48 is brought into contactwith the master workpiece 92. Described more specifically, if thecutting point of the cutting tool 48 is wet with the cutting fluid whenthe cutting tool 48 approaches to the master workpiece 92, the cuttingfluid sticking to the cutting point of the cutting tool 48 is broughtinto contact with the master workpiece 92 before the cutting pointitself is brought into contact with the master workpiece 92. In thisinstance, the cutting tool 48 is likely to be electrically connected tothe master workpiece 92 by the cutting fluid which has commonly has acertain degree of electrical conductivity. The electrical connection ofthe cutting tool 48 and the master workpiece 92 via the cutting fluidcauses the first circuit 96 to be semi-closed, thereby allowing anelectric current to flow through both the first and second circuits 96,98 from the DC power source 104, resulting in an increase in a value ofthe flowing electric current detected by the current detector 100.However, a value of electric resistance of the cutting fluid is heldlarger than 500 Ω in this embodiment, although the resistance value ofthe cutting fluid generally varies depending upon its component. Thus,the increase in the detected value of the flowing electric current inthis instance is relative small, and accordingly the detected value doesnot exceed a predetermined third value which is larger than theabove-described second value. This means that the computer 80 does noterroneously determines that the cutting point of the cutting tool 48 isin contact with the outer circumferential surface of the masterworkpiece 92.

[0187] When the cutting point of the cutting tool 48 is actually broughtinto contact with the outer circumferential surface of the masterworkpiece 92, the first circuit 96 is completely closed, the electriccurrent is fully allowed to flow through the first and second circuits96, 98 from the DC power source 104, resulting in an abrupt increase inthe value of the electric current detected by the current detector 100.Since the resistance value of the resistance R2 (provided in the firstcircuit 96) is adapted to be much smaller than that of the resistance R1(provided in the second circuit 98) in the present embodiment, asdescribed above, the detected value of the electric current is abruptlyincreased. The computer 80 determines that the cutting point of thecutting tool 48 has been brought into contact with the outercircumferential surface of the master workpiece 92, when confirming thatthe detected current value has exceeded the predetermined third value asa result of its abrupt increase.

[0188] Upon determination that the cutting tool 48 is in contact withthe master workpiece 92, the computer 80 commands the X-axis movementdevice 66 (and additionally the Z-axis movement device 56 if it is alsobeing activated) to stop the movement of the cutting tool 48, whilecalculating the current position of the cutting point of the cuttingtool 48 in the X-axis direction, on the basis of an output provided bythe rotary encoder 74. This current position of the cutting point of thecutting tool 48 may be represented by, for example, a distance overwhich the cutting tool 48 has been moved, as viewed in the X-axisdirection, from a so-called “machine home position” until the contact ofthe cutting tool 48 with the master workpiece 92. Data representative ofthe calculated position of the cutting point of the cutting tool 48 arestored, as contact-position data, in a contact-position memory of theRAM 84. If there are previous contact-position data with respect to thesame cutting tool 48 in the contact-position memory, the previouscontact-position data are replaced with the new contact-position data.For example, the above-described distance from the machine home positionto the contact position is gradually increased with an increase in theamount of wear of the cutting point of the cutting tool 48, and is alsolikely to be changed depending upon various factors. By renovating thecontact-position data, it is possible to prevent a deterioration in adimensional accuracy of the product produced in the operation.

[0189] This step of determining, as the contact position, the positionof the cutting tool 48 relative to the master workpiece 92 when thetransition from the open state to the closed state of the first circuit96 is detected, is referred to as a “contact-position determining step”.This step may be also referred to as a “contact-position-data storingstep”, since the contact position is stored in the computer 90. Thisstep is advantageously implemented, for example, each time apredetermined time has been passed or a predetermined number of producthave been produced, for preventing the dimensional accuracy of theproduct from being affected by various factors such as wear of thecutting insert 112 and deformation of the main body portion of the NClathe due to a temperature change in the main body itself. This step maybe implemented also at predetermined points of time, e.g., at the timeof change of the operator or at the time when the NC lathe starts to beoperated in each day, or when needed, e.g., before the first workpieceis machined after the cutting tool 42 or the cutting insert 112 has beenreplaced with a new one.

[0190] In the present embodiment, the data representative of theposition of the cutting tool 48 upon its contact with the masterworkpiece 92 are stored in the computer 90. However, some otherpositional data, in addition to or in place of the contact-positiondata, may be stored in the computer 90. For example, it is also possibleto locate the position the axis of the main spindle 22 about which themain spindle 22 is to be rotated, on the basis of the contact positionof the cutting tool 48 and a known diameter of the master workpiece 92.This position of the rotary axis may be stored, as a reference positionin the X-axis direction, in a reference-position memory of the RAM 84.Further, the cutting tool 48 may be brought into contact with an endface of the master workpiece 92, if needed, by moving cutting tool 48toward the master workpiece 92 in the axial direction of the masterworkpiece 92, so that the position of the cutting tool 48, in which thecutting tool 48 is brought into at its cutting point with the end faceof the master workpiece 92, is stored, as a reference position in theZ-axis direction.

[0191] After the positional data have been stored in the correspondingmemory of the RAM 84, the master workpiece 92 is replaced with theworkpiece 32, which in turn is attached to the chuck 28. The workpiece32 is machined or cut by the cutting tool 48 in accordance with thecurrently stored or renovated positional data, and is formed into aproduct having a high degree of dimensional accuracy. Even if thecutting point of the cutting tool 48 is not accurately aligned with theheight of the rotary axis of the main spindle 22, namely, even if thecutting point of the cutting tool 48 does not lie on a line which passesthe axis and which is parallel to the X-axis direction, the dimensionalaccuracy of the product is not deteriorated as long as an amount ofdeviation of the cutting point from this line is not considerably large.That is, the disalignment of the cutting point of the cutting tool 48with respect to the height of the axis of the main spindle 22 does notseriously affect the dimensional accuracy of the product because thepositional data are prepared by using the cutting tool 48 which is usedfor forming the workpiece into the product. On the other hand, where atouch sensor is used for preparing the positional data, the dimensionalaccuracy of the product is deteriorated if a touch probe of the touchsensor deviates from the above-described line by an amount differentfrom the amount by which the cutting point of the cutting tool deviatesfrom the line. In the present embodiment, it is possible to avoid such adeterioration in the dimensional accuracy due to the disalignment of thecutting point with the respect of the axis of the main spindle 22.

[0192] The computer 80 keeps monitoring or reading the value of theelectric current detected by the current detector 100 while the cuttingtool 48 is moved toward the workpiece 32 in an initial stage of theoperation. In this instance, the computer 80 determines that there is apossibility that the cutting insert 112 has been damaged, if thedetected value of the electric current does not exceed the predeterminedthird value even after the cutting tool 48 has been moved to apredetermined position in which the cutting tool 48 should be broughtinto contact with the workpiece 32, namely, if the detected electriccurrent value does not exceed the predetermined third value even whilethe relative position of the cutting tool 48 and the workpiece 32detected by the rotary encoders 72, 74 satisfies a positional conditionrequired for the contact of the two members 48, 32. After thedetermination of the possible damage of the cutting insert 112 has beenmade, the movement devices 56, 66 are commanded to immediately stop themovement of the cutting tool 48 toward the workpiece 32, and at the sametime an alarm light or an alarm buzzer is activated to inform theoperator that the cutting insert 112 has been possibly damaged.

[0193] Even after the workpiece 32 has started to be cut by the cuttingtool 48, the computer 80 still keeps monitoring or reading the electriccurrent value detected by the current detector 100. If the detectedelectric current value is reduced to be equal to or smaller than apredetermined fourth value during the cutting operation, the computer 80determines that there is a possibility that the cutting insert 112 hasbeen damaged, and then commands the movement devices 56, 66 to move thecutting tool 48 away from the workpiece 32. The cutting operation isimmediately suspended, and the alarm light or buzzer is activated. Thepredetermined fourth value is set to be larger than the value of theelectric current flowing through the cutting tool 48 and the workpiece32 when the two members are shorted to each other by the cutting fluid.In either of this actual cutting stage and the above-described initialstage of the operation, the operator can take check the cutting tool 48or cutting insert 112 in response to the activation of the alarm lightor buzzer, so as to take a necessary procedure. In this embodiment, thepossible damage of the cutting tool 48 is easily detected.

[0194] Where a plurality of workpieces 32 are successively cut, it ispossible to measure a dimension of the formed product, for example, eachtime a predetermined number of products have been formed. If themeasured dimension of the product is within a predetermined tolerance,the successively cutting operation is continued without modifying theabove-described positional data or contact-position data currentlystored in the RAM 84 of the computer 80. If the measured dimension ofthe product is not within a predetermined tolerance, the currentlystored data are modified in view of an amount of deviation of themeasured dimension from the target dimension, so that the successivelycutting operation is continued with the thus modified data.

[0195] As is apparent from the above descriptions, a portion of thecomputer 80, which portion is assigned to detect the contact position inwhich the cutting point of the cutting tool 48 is brought into contactwith the master workpiece 92, constitutes a control device as defined inmode (19) which is described above in SUMMARY OF THE INVENTION. Thefirst circuit 96 and the current detector 100 constitute a contactdetecting device as defined in mode (18). A portion of the computer 80which portion is assigned to execute the above-described checking stepcooperates with the second circuit 98 to constitute a checking device asdefined in mode (18). A portion of the computer 80 which portion isassigned to execute the above-described contact determining stepconstitutes a contact determining device as defined in mode (18). It isnoted that the term “cutting point” may be interpreted to correspond toa portion of the cutting edge of the cutting blade.

[0196] Referring next to FIG. 5, there will be described acutting-blade-position controlling apparatus which is constructedaccording to a second embodiment of this invention. While the fixedresistor 106 in which the terminals 116, 118 are disposed in theopposite ends of the resistive body 114 is used in the above-describedfirst embodiment, the fixed resistor 106 is replaced with a resistivelayer 170 in this second embodiment. As schematically shown in of FIG.5, the resistive layer 170 is interposed between a cutting tool 172 anda turret 174 which are provided by respective electrically conductivemembers each having a high degree of electrical conductivity. Theresistive layer 170 may be formed on a surface of either one of thecutting tool 172 and the turret 174 so as to be bonded to the surface,or may be formed of an independent sheet so as to be simply interposedbetween the two members 172, 174 without being bonded to any one of thetwo members 172, 174. In any one of these cases, the cutting tool 172and the turret 174 have to be fixed relative to each other, with theresistive layer 170 being interposed between the two members 172, 174. Afixing device for the fixing the two members 172, 174 has to be adaptedto inhibit the two members 172, 174 from having an electrical continuitywith each other.

[0197] The resistive layer 170 may be formed of a synthetic resin,ceramic or other electrically insulating material, into which metallicpowders or other electrically conductive powders are mixed. Theformation of the resistive layer 170 on the surface of the conductivemember may be achieved, for example, in accordance with a PVD method, aCVD method, a spray-forming method or a coating method. Where aresistive layer is formed on a cutting tool, a cutting-tool holdingmember or other member which preferably has a high degree of hardness,it is preferable that the resistive layer is formed in accordance withthe PVD method since the conductive layer is heated at a lowertemperature in a practice of the PVD method than in a practice of theCVD method.

[0198]FIG. 6 shows a third embodiment of the invention, which isidentical to the second embodiment shown in FIG. 5 except that thecutting point of the cutting tool 172 is brought into contact with anouter circumferential surface of the workpiece 32 which is to be cut.Where the workpiece 32 has a measured or otherwise known diameter, theworkpiece 32 can be formed into a product having a desired diameter, bypositioning the cutting tool 172 in a predetermined radial position thatis determined on the basis of the known diameter of the workpiece 32,the desired diameter of the product and the contact position in whichthe cutting tool 172 has been contact with the outer circumferentialsurface of the workpiece 32 having the known diameter. Namely, thecutting tool 172 is moved relative to the workpiece 32 over apredetermined distance in the axial direction, after having being movedto the above-described predetermined radial position.

[0199]FIG. 7 shows a fourth embodiment of the invention, which isidentical to the second and third embodiments shown in FIGS. 5 and 6except that the cutting point of the cutting tool 172 is brought intocontact with an outer circumferential surface of a main body 178 of thechuck 28. In this fourth embodiment, the above-described radial positionof the cutting tool 172 is determined on the basis of a diameter of themain body 178 of the chuck 28, the desired diameter of the product andthe contact position in which the cutting tool 172 has been contact withthe outer circumferential surface of the main body 178 of the chuck 28.In this embodiment, the main body 178 of the chuck 28 constitutes theabove-described object in the form of a reference portion of the machinetool. It is preferable that the object (such as the mater workpiece 92,the workpiece 32 and the main body 178 of the chuck 28) which is to bebrought into contact with the cutting tool has to have a high degree ofroundness and also a high degree of coaxial relation with the mainspindle 22 so as to be rotatable without its run-out.

[0200]FIG. 8 shows a fifth embodiment of the invention, in which a touchprobe 180, as well as the cutting tool 48, is attached to the turret 46.A contact detecting circuit, which is similar to the above-describedcontact detecting circuit 94, is provided to incorporate therein thetouch probe 180. The use of the touch probe 180 makes it possible tolocate the position of the rotary axis of the workpiece 32 or masterworkpiece 92 which is fixed to the chuck 28, by bringing a sphericalcontact end of the touch probe 180 in contact with two portions of anouter circumferential surface of the workpiece 32 or master workpiece 92which portions are diametrically opposed to each other. That is, anintermediate point between the detected two portions of the outercircumferential surface of the workpiece 32 or master workpiece 92 canbe determined as the position of the rotary axis. Such data obtained bythe touch probe 180 can be used in the control of the movement of thecutting tool 48, if a positional relationship between the cutting tool48 and the touch probe 180, i.e., a relative position of the two members48, 180 is known. This relative position can be known, for example, bybringing the cutting point of the cutting tool 48 and the sphericalcontact end of the touch probe 180 into contact with the same portion ofthe workpiece 32 or master workpiece 92. The relative position of thetwo members 32, 92 can be calculated on the basis of a distance overwhich the turret 46 is moved between a cutting-tool contact position inwhich the cutting tool 48 is brought into contact with theabove-described same portion and a touch-probe contact position in whichthe touch probe 180 is brought into contact with the above-describedsame portion.

[0201] It is also possible to cover a surface of the cutting blade, thecutting tool or the cutting-tool holding member, with an electricallyinsulating coating. FIG. 9 shows a sixth embodiment of the invention inwhich the tool body 110 of the cutting tool 48 is covered at its surfacewith an electrically insulating film or coating 181. The provision ofthe insulating coating 181 on the surface of the tool body 110 iseffective to prevent the turret 46 and the tool body 110 of the cuttingtool 48 from being shorted at their mutually adjacent portions to eachother by a cutting fluid, cutting chips or other substance sticking tothe surfaces of the turret 46 and the tool body 110 of the cutting tool48, namely, prevent electrical connection between the mutually adjacentportions through a by-passing passage which is formed of the stickingsubstance and which is positioned in parallel with the fixed resistor106 of the second circuit 98. Therefore, this arrangement advantageouslyavoids' an erroneous determination that the cutting insert 112 of thecutting tool 48 has been brought into contact with the workpiece 32 ormaster workpiece 92. Such an erroneous determination could be caused byan increase in the detected value of the electric current as a result ofthe electrical connection between the turret 46 and the cutting tool 48,which is effectively prevented by the insulating coating 181 in thissixth embodiment. It is noted that the electrically insulating coating181 is formed on the surface of the tool body, 110 of the cutting tool48 such that the insulating coating 181 covers the surface of the toolbody 110 except portions of the surface which are to be held in contactwith the cutting blade 112, the terminal 116 of the resistor 106 and thecontact member 162.

[0202] The sticking of the cutting fluid and the cutting chips to thesurfaces of the members (e.g., the cutting blade, the cutting tool, thecutting-tool holding member, the workpiece, the master workpiece, theworkpiece, and the reference portions) can be prevented also byimplementing a cleaning step in which the cutting fluid and the cuttingchips are removed from the members before the implementation of thecontact determining step. That is, this cleaning step also serves toprevent the above-described erroneous determination that the cuttingtool 48 has been brought into contact with the workpiece 32 or masterworkpiece 92. The cleaning step may be substituted for theabove-described provision of the electrically insulating layer on thesurface of the cutting tool, or alternatively may be implementedtogether with the provision of the insulating layer. In the latter case,the above-described erroneous determination can be more surely avoided.

[0203] Referring next to FIG. 10, there will be described acutting-blade-position controlling apparatus which is constructedaccording to a seventh embodiment of this invention. Thiscutting-blade-position controlling apparatus is built in a machiningcenter, a milling machine or a boring machine. In this seventhembodiment, a reference portion 184 which is fixed to a main structure182 of the machining center, milling machine or boring machineconstitutes the above-described object to be brought in contact with thecutting tool. The reference portion 184 preferably has three referencesurfaces 186, 188, 190 which are held perpendicular to the X-axis,Y-axis and Z-axis directions, respectively. A rotary cutting tool 192 isbrought into contact at its cutting point with each of these surfaces186, 188, 190, for thereby detecting a contact position in which thecutting tool 192 is brought into contact with each of the surfaces 186,188, 190. The thus detected contact position is used as a referenceposition in the corresponding one of the X-axis, Y-axis and. Z-axisdirections.

[0204] The rotary cutting tool 192 may be brought into contact with thereference surfaces 186, 188, 190, with or without the cutting tool 192being rotated. However, it is preferable that the cutting tool 192 isbeing rotated upon its contact with the X-axis-direction referencesurface 186 or the Y-axis-direction reference surface 188, particularly,where the cutting tool 192 has a plurality of cutting teeth or edgeswhose respective radial distances from the rotary axis of the cuttingtool 192 are different from each other. In this instance, it is possibleto determine, as the contact position, the relative position in whichthe reference surface 186 or 188 is brought into contact with one of thecutting edges which has a larger radial distance from the rotary axisthan the other cutting edges. When the cutting tool 192 is brought intocontact with the Z-axis-direction reference surface 190, it ispreferable that the cutting tool 192 is being rotated if the cuttingedges of the cutting tool 192 having respective lower end portions whoseaxial heights are different from each other. In this instance, it ispossible to determine, as the contact position, the relative position inwhich the reference surface 190 is brought into contact with one of thelower end portions of the respective cutting edges which has a smallerheight than the lower end portions of the other cutting edges. It isnoted that if a workpiece 193 also has three reference surfaces whichare perpendicular to the X-axis, Y-axis and Z-axis directions, thecutting tool 192 may be brought into contact with the workpiece 193 inplace of the reference portion 184.

[0205] Also in this seventh embodiment of FIG. 10, an electricresistance is provided between the cutting tool and a main body portionof the machine tool. Described specifically, a resistive layer 198 isprovided between the blade holding member in the form of a tool holder194 holding the rotary cutting tool 192, and a tool spindle 196 to whichthe tool holder 194 is attached. The resistive layer 198 is formed on aninner circumferential surface of a taper hole 200 of the tool spindle196, so that the resistive layer 198 is interposed between the innercircumferential surface of the taper hole 200 and an outercircumferential surface of a taper shank portion of the tool holder 194.Also in this arrangement, a fixing device (e.g., a collet chuck, drawbar) for removably fixing the tool holder 194 to the tool spindle 196has to be adapted to inhibit the tool holder 194 and the tool spindle196 from having an electrical continuity with each other.

[0206]FIG. 11 shows an eighth embodiment of the invention which isdifferent from the seventh embodiment of FIG. 10, in that an electricresistance is provided between the main body portion of the machine tooland a workpiece holding member for holding a workpiece. That is, aresistive layer 216 is interposed between the main body in the form of amain structure 214 and the workpiece holding member in the form of a jigor fixture 212 to which a workpiece 210 is fixed.

[0207]FIG. 12 shows a ninth embodiment of the invention which isidentical to the eight embodiment of FIG. 11, in that an electricresistance is provided between the main body portion of the machine tooland the workpiece holding member for holding a workpiece. However, theresistive layer 216 is replaced with a fixed resistor 226 which isprovided between the main body of the machine tool in the form of a worktable 220 that is movable relative to the main structure by a movementdevice, and the workpiece holding member in the form of a jig or fixture224 to which a workpiece 222 is fixed. Like the above-described fixedresistor 106 which is best shown in FIG. 3, the fixed resistor 226,together with a compression coil spring 227 is received in a generallycylindrical insulating body 228 which is made of an electricallyinsulating material, such that the resistor 226 is movable relative tothe cylindrical insulating body 228 in an axial direction of thecylindrical housing 228.

[0208] Referring next to FIGS. 13 and 14, there will be described acutting-blade-position controlling apparatus which is constructedaccording to a tenth embodiment of this invention. Thecutting-blade-position controlling apparatus of this tenth embodiment issubstantially identical to that of the above-described first embodimentwhich are shown in FIGS. 1-4, except that the lathe cutting tool 48 isreplaced with a lathe cutting tool 348. In the following description ofthis second embodiment, the same reference numeral as used in the firstembodiment will be used to identify the elements which are the same asor similar to those in the first embodiment. No redundant description ofthese elements will be provided, in the interest of simplification ofthe description.

[0209] The lathe cutting tool 348 includes a tool body 410, and areplaceable cutting insert 412 which is attached to the tool body 410 byclamping means (not shown). The cutting insert 412 is made of anelectrically conductive material having a high degree of electricalconductivity, and is covered at its entirety with a conductive film orcoating 413, as shown in FIG. 14. The conductive coating 413 is formedof a material having a suitable degree of electric resistance such as amixture of an electrically insulating material (e.g., a synthetic resinand ceramic) and an electrically conductive material (e.g. a metallicpowder). The conductive coating 413 formed of such a suitable materialis bonded to the entire surface of the cutting insert 412 such that theconductive coating 413 has a constant and accurate thickness over theentire surface of the cutting insert 412. The bonding of the conductivecoating 413 to the surface of the cutting insert 412 may be achieved,for example, in accordance with a PVD method, a CVD method, aspray-forming method or a coating method. It is preferable that theconductive coating 413 is formed in accordance with the PVD method sincethe cutting insert 412 is heated at a lower temperature in a practice ofthe PVD method than in a practice of the CVD method.

[0210] It is preferable but not essential that the conductive coating413 has a thickness constant over its entirety, as long as the thicknessof a portion of the coating 413 positioned in the cutting edge of thecutting insert 412 is known. It is possible to reduce a possibility ofchipping of the cutting insert 412 or damage of the master workpiece 92,by increasing the thickness of the conductive coating 413. However, thethickness of the conductive coating 413 may be reduced to such an extentthat the thickness is still large enough to avoid complete destructionof the conductive coating 413 when the relative movement of the cuttinginsert 412 and the master workpiece 92 is stopped in response to thedetection of the contact of the two members 92, 412. Even if thethickness of the coating 413 is reduced such an extent that thethickness is no longer large enough to avoid the complete destruction ofthe coating 413, it is possible to still enjoy the technical advantagesprovided by the present invention, as long as the reduced thickness ofthe coating 413 is larger enough to avoid damages of the cutting insert412 and the master workpiece 92.

[0211] In the cutting-blade-position controlling apparatus of this tenthembodiment of this invention, a contact of the cutting tool 348 with themaster workpiece 92 is detected by a contact detecting circuit 394 whichis connected to the control device 78. In the NC lathe equipped with thecontact detecting circuit 394 which is constructed as described above,an operation for cutting the workpiece 32 is carried out as follows.

[0212] The operation is initiated by attaching the cutting tool 348 andthe master workpiece 92 to the turret 46 and the chuck 28, respectively,so that the first and second circuits 96, 98 are formed. In the firstcircuit 96, the cutting tool 348, the current detector 100, the DC powersource 104, the main body portion of the NC lathe and the masterworkpiece 92 are arranged in series. In the second circuit 98, thecutting tool 348, the current detector 100, the DC power source 104, themain body portion of the NC lathe and the fixed resistor 106 arearranged in series. An electric resistance denoted by a sign R1 in FIG.13 represents an electric resistance acting on the second circuit 98Thus, the electric resistance R1 actually includes not only the fixedresistor 106 but also an electric resistance provided by the turret 46and other parts of the main body portion of the NC lathe. However, theelectric resistance R1 may be interpreted to represent the fixedresistor 106, since the electric resistance provided by the main bodyportion of the NC lathe has a considerably small resistance value. It ispreferable that the fixed resistor 106 has a resistance value which isdetermined depending upon the value of the electric resistance providedby the main body portion of the NC lathe and also a value of an electricresistance provided by a cutting fluid used in the cutting operation,such that the resistance value of the resistor 106 is not smaller than50 Ω and is smaller than 500 Ω. In the present embodiment, theresistance value of the resistor 106 is set to be 250 Ω.

[0213] An electric resistance denoted by a sign R2 represents anelectric resistance acting on the first circuit 96. The electricresistance R2 represents an electric resistance provided by the mainspindle 22 and the other parts of the main body portion of the NC lathe.Electric resistances denoted by signs R3, R4 represent electricresistances acting on the first circuit 96. The electric resistance R3represents an electric resistance provided by a portion of theconductive coating 413 which portion is interposed between the cuttinginsert 412 and the tool body 410. The electric resistance R4 representsan electric resistance provided by a portion of the conductive coating413 which portion is interposed between the cutting insert 412 and themaster workpiece 92 upon contact of the cutting insert 412 and themaster workpiece 92 with each other via the conductive coating 413. Eachof the values of the electric resistances R1, R3 and R4 is adapted to bemuch larger than that of the electric resistance R2. As to arelationship between the values of the respective electric resistancesR3 and R4, the value of the electric resistance R3 is commonly muchsmaller than that of the electric resistance R4 due to a difference incross sectional area between conductive passages respectively providedby the above-described two portions of the conductive coating 413. Thus,the relationship can be commonly expressed by the flowing inequality:

R4>>R3

[0214] Further, the value of the electric resistance R4 is preferablyadapted to be substantially equal to or smaller than that of theelectric resistance R1. This is because it is preferable the value ofthe electric current is doubled or more upon the contact of the cuttinginsert 412 with the master workpiece 92 via the conductive coating 413,namely, upon transition from a closed state of the first circuit 96 inwhich the electric current is allowed to flow through only the secondcircuit 98, to an open state of the first circuit 96 in which theelectric current is allowed to flow through not only the second circuit98 but also the first circuit 96. The transition from the closed stateto the open state can be easily detected with an increase in an amountof change in the value of the flowing electric current upon thetransition. In the present embodiment, the value of the electricresistance R4 is adapted to be about 10-50% of that of the electricresistance R1. Thus, in the present embodiment, the relationship amongR1, R2, R3 and R4 can be expressed by the flowing inequality:

R1>R4>>R3>>R2

[0215] In an initial stage of the operation in which a cutting point ofthe cutting insert 412, i.e., a distal end or cutting point of thecutting tool 348 is in a position distant from the master workpiece 92,the first circuit 96 is held in open state in which the first circuit 96is open. In this embodiment of the invention, even when the cuttingpoint of the cutting tool 348 is separated from the master workpiece 92,a small amount of electric current flows through the second circuit 98which is held closed irrespective of whether the cutting tool 348 is incontact with the master workpiece 92 or not. This electric current isdetected by the current detector 100, but the detected value of thecurrent is small due to the electric resistance R1 having a largeresistance value. However, the detected value is obviously larger thanzero, and is not smaller than a predetermined first value and not largerthan a predetermined second value that is larger than the predeterminedfirst value. The computer 80 of the control device 78 operates toexecute a program for reading the detected value of the electriccurrent, and then determines that the contact detecting circuit 394 isin a normal condition if the read value of the electric current is notsmaller than the predetermined first value and not larger than thepredetermined second value. The normal condition is interpreted to meansa condition in which an electric voltage is applied between the cuttingpoint of the cutting tool 348 and the master workpiece 92, for enablingthe circuit 394 to detect a contact of the cutting tool 348 and themaster workpiece 92 when the contact is actually achieved. If it is notdetermined at this checking step that the contact detecting circuit 394is in the normal condition, the computer 80 inhibits the movementdevices 56, 66 from carrying out relative movement of the cutting tool348 and the master workpiece 92 toward to each other. This arrangementis effective to prevent failure to detect the actual contact of thecutting tool 348 and the master workpiece 92, making it possible toavoid a dangerous situation in which at least one of the cutting tool348 and the master workpiece 92 is further moved toward each other evenafter their actual contact, without a risk of damaging the cutting tool348, the master workpiece 92 or holders holding the cutting tool 348 andthe master workpiece 92.

[0216] If it is determined at the checking step that the contactdetecting circuit 394 is in the normal condition, the computer 80 of thecontrol device 78 operates to execute a program for controlling theZ-axis and X-axis motors 52, 62 of the respective Z-axis and X-axismovement devices 56, 66, such that the cutting tool 348 is moved towardthe master workpiece 92, so as to be brought into proximity to an outercircumferential surface of the master workpiece 92. The cutting tool 348is moved toward the master workpiece 92 at a high feed rate while theyare distant from each other. The cutting tool 348 is then decelerated ata predetermined decelerating position which is near to the masterworkpiece 92. Thus, the cutting tool 348 is brought into proximity tothe outer circumferential surface of the master workpiece 92, with itsapproaching movement at a low feed rate. It is preferable thisapproaching movement of the cutting tool 348 to the outercircumferential surface of the master workpiece 92 is carried out bymoving the cutting tool 348 only in the radial direction of the masterworkpiece 92, i.e., only in the X-axis direction.

[0217] Where a cutting fluid is used in the operation, there is apossibility that the cutting point of the cutting tool 348 is broughtinto connection with the master workpiece 92 via the cutting fluidbefore the cutting point of the cutting tool 348 is brought into contactwith the master workpiece 92. Described more specifically, if thecutting point of the cutting tool 348 is wet with the cutting fluid whenthe cutting tool 348 approaches to the master workpiece 92, the cuttingfluid sticking to the cutting point of the cutting tool 348 is broughtinto contact with the master workpiece 92 before the cutting pointitself is brought into contact with the master workpiece 92. In thisinstance, the cutting tool 348 is likely to be electrically connected tothe master workpiece 92 by the cutting fluid which has commonly has acertain degree of electrical conductivity. The electrically connectionof the cutting tool 348 and the master workpiece 92 via the cuttingfluid causes the first circuit 96 to be semi-closed, thereby allowing anelectric current to flow through both the first and second circuits 96,98 from the DC power source 104, resulting in an increase in a value ofthe flowing electric current detected by the current detector 100.However, since a value of electric resistance of the cutting fluid isheld larger than 500 Ω in this embodiment, the increase in the detectedvalue of the flowing electric current in this instance is relativesmall. The detected value accordingly does not exceed a predeterminedthird value which is larger than the above-described second value. Thismeans that the computer 80 does not erroneously determines that thecutting point of the cutting tool 348 is in contact with the outercircumferential surface of the master workpiece 92.

[0218] When the cutting point of the cutting tool 348 or the cuttinginsert 412 is actually brought into contact with the outercircumferential surface of the master workpiece 92, the first circuit 96is completely closed. In this instance, the cutting tool 348 is incontact with the master workpiece 92 via the conductive coating 413which covers the cutting insert 412, the electric current flows throughthe electric resistances R3, R4, R2 which are arranged in series in thefirst circuit 96. Since the relationship among R2, R3 and R4 isexpressed by the inequality R4>>R3>>R2, namely, the value of theelectric resistance R4 is the largest among the values of the threeelectric resistances R2, R3, R4, the value of the electric currentflowing through the first circuit 96 generally depends on the value ofthe electric resistance R4, which is adapted to be about 10-50% of thatof the electric resistance R1. Therefore, the value of the electriccurrent detected by the current detector 100 is abruptly increased suchthat the detected value after the contact of the cutting toll 348 withthe master workpiece 92 corresponds to about 2-10 times the detectedvalue before the contact. The computer 80 determines that the cuttingpoint of the cutting tool 348 has been brought into contact with theouter circumferential surface of the master workpiece 92, whenconfirming that the detected current value has exceeded thepredetermined third value as a result of its abrupt increase. This stepcorresponds to a contact determining step. In the present embodiment inwhich the cutting insert 412 and the master workpiece 92 are broughtinto contact with each other via the conductive coating 413 that is madeof material softer than those of the two members 92, 412, it is possibleto minimize a risk for damage of these two members 92, 412.

[0219] In the above-described contact determining step, there is apossibility that the cutting tool 348 and the master workpiece 92 areconnected via a cutting fluid even before the two members 92, 348 arebrought into contact with each other. In such a case, if the cuttingfluid has an electrical conductivity, the electric current is likely tobe flow through the cutting fluid, thereby possibly causing an erroneousdetermination that the cutting tool 348 has been brought into contactwith the mater workpiece 92. For preventing such an erroneousdetermination, the predetermined third value is preferably set to besufficiently larger than a value of the electric current flowing throughthe cutting fluid.

[0220] In the present embodiment, prior to the implementation of thecontact determining step, there is implemented a cleaning step in whicha compressed air is blasted from a nozzle 468, for removing the cuttingfluid and the cutting chips sticking to surfaces of the cutting tool 348and the master workpiece 92. In this cleaning step, it is preferable toclean surfaces of the turret 46 and the chuck 28, in addition to thecutting tool 348 and the master workpiece 92. The implementation of thecleaning step is effective to prevent the above-described erroneousdetermination.

[0221] Upon determination that the cutting tool 348 is in contact withthe master workpiece 92, the computer 80 commands the X-axis movementdevice 66 (and additionally the Z-axis movement device 56 if it is alsobeing activated) to stop the movement of the cutting tool 348, whilecalculating the current position of the cutting point of the cuttingtool 348 in the X-axis direction, on the basis of an output provided bythe rotary encoder 74. Data representative of the calculated position ofthe cutting point of the cutting tool 348 are stored, ascontact-position data, in a contact-position memory of the RAM 84. Thisstep corresponds is referred to as a “contact-position determiningstep”, or may be also referred to as a “contact-position-data storingstep”.

[0222] In the present embodiment, the data representative of theposition of the cutting tool 348 upon its contact with the masterworkpiece 92 are stored in the computer 90. However, some otherpositional data, in addition to or in place of the contact-positiondata, may be stored in the computer 90. For example, it is also possibleto locate the position the axis of the main spindle 22 about which themain spindle 22 is to be rotated, on the basis of the contact positionof the cutting tool 348 and a known diameter of the master workpiece 92.This position of the rotary axis may be stored, as a reference positionin the X-axis direction, in a reference-position memory of the RAM 84.Further, the cutting tool 348 may be brought into contact with an endface of the master workpiece 92, if needed, by moving cutting tool 348toward the master workpiece 92 in the axial direction of the masterworkpiece 92, so that the position of the cutting tool 348, in which thecutting tool 348 is brought into at its cutting point with the end faceof the master workpiece 92, is stored, as a reference position in theZ-axis direction.

[0223] After the positional data have been stored in the correspondingmemory of the RAM 84, the master workpiece 92 is replaced with theworkpiece 32, which in turn is attached to the chuck 28. The workpiece32 is machined or cut by the cutting tool 348 in accordance with thecurrently stored or renovated positional data, and is formed into aproduct having a high degree of dimensional accuracy.

[0224] The computer 80 keeps monitoring or reading the value of theelectric current detected by the current detector 100 while the cuttingtool 348 is moved toward the workpiece 32 in an initial stage of theoperation, so as to detect a contact of the cutting tool 348 with theworkpiece 32 via the conductive coating 413, and then detect a directcontact of the cutting tool 348 with the workpiece 32 after theconductive coating 413 has been destroyed. The value of the electriccurrent detected by the current detector 100 exceeds the above-describedpredetermined third value when the cutting tool 348 is brought intocontact with the workpiece 32 via the conductive coating 413, when thecutting tool 348 is brought into contact with the master workpiece 92via the conductive coating 413. Thus, the computer 80 can detect thecontact of the cutting tool 348 with the workpiece 32 via the conductivecoating 413, on the basis of the fact that the detected value of theelectric current has exceeded the predetermined third value. When thecutting tool 348 is brought into direct contact with the workpiece 32after the destruction of the conductive coating 413, the detected valueof the electric current is further increased since the value of theelectric resistance R4 is zeroed. In this instance, the value of theelectric resistance acting on the first circuit 96 corresponds to thesum of the values of the electric resistances R2, R3 (R2+R3). This valueis considerably smaller than the sum of the values of the electricresistances R2, R3, R4 (R2+R3+R4), i.e., the value of the electricresistance acting on the first circuit 96 when the cutting tool 348 isin contact with the workpiece 32 via the conductive coating 413. Thatis, the detected value of the electric current is increased upon thedirect contact of the cutting tool 348 with the workpiece 32, by anamount corresponding to the reduction in the value of the electricresistance acting on the first circuit 96. Thus, the computer 80 candetect the direct contact of the cutting tool 348 with the workpiece 32,on the basis of the fact that the detected value of the electric currenthas been further increased

[0225] When the cutting tool 348 and the workpiece 32 are in directcontact with each other without intervention of the conductive coating413 therebetween, the value of the flowing electric current depends onthe sum of the values of the electric resistances R2, R3, as describedabove. However, this value of the flowing electric current can beconsidered to depend on almost only the value of the electric resistanceR3 which corresponds to the electric resistance provided by the portionof the conductive coating 413 interposed between the cutting insert 412and the tool body 410, since the value of the resistance R2 isconsiderably smaller than the value of the resistance R3. The value ofthe electric resistance R3 is almost proportional to the cross sectionalarea of the conductive passage provided by the above-described portionof the conductive coating 413. Therefore, it can be determined that thecutting insert 412 is a currently required cutting insert, i.e., acorrect cutting insert in accordance with a cutting operation program,if the value of the electric current detected by the current detector100 upon the direct contact of the cutting insert 412 with the workpiece 32 is held in a range which has been set for the correct cuttinginsert. It can be determined that the cutting insert 412 is not acurrently required cutting insert and that the cutting insert 412 (orthe cutting tool 348) has been erroneously attached to the tool body 410(or the turret 46), if the detected value of the electric current uponthe direct contact is not held in the range which has been set for thecorrect cutting insert. In the latter case, the rotation of the mainspindle 22 and the movement of the cutting tool 348 are stopped afterthe cutting tool 348 has been separated from the workpiece 32 by apredetermined distance, while at the same time the operator is informedby activations of an alarm light or buzzer and an alarm indicator thatthe cutting insert 412 is not a correct cutting insert. This stepcorresponds to a blade-selection checking step.

[0226] Further, the computer 80 determines that there is a possibilitythat the cutting insert 412 has been damaged, if the detected value ofthe electric current does not exceed the predetermined third value evenafter the cutting tool 348 has been moved to a predetermined position inwhich the cutting tool 348 should be brought into contact with theworkpiece 32, namely, if the detected electric current value does notexceed the predetermined third value even while the relative position ofthe cutting tool 348 and the workpiece 32 detected by the rotaryencoders 72, 74 satisfies a positional condition required for thecontact of the two members 348, 32. After the determination of thepossible damage of the cutting insert 412 has been made, the movementdevices 56, 66 are commanded to immediately stop the movement of thecutting tool 348 toward the workpiece 32, and at the same time the alarmlight or buzzer and the alarm indicator are activated to inform theoperator that the cutting insert 412 has been possibly damaged. Thisstep corresponds to a first breakage determining step.

[0227] Even after the workpiece 32 has started to be cut by the cuttingtool 348, the computer 80 still keeps monitoring or reading the electriccurrent value detected by the current detector 100. If the detectedelectric current value is reduced to be equal to or smaller than apredetermined fourth value during the cutting operation, the computer 80determines that there is a possibility that the cutting insert 412 hasbeen damaged, and then commands the movement devices 56, 66 to move thecutting tool 348 away from the workpiece 32. The cutting operation isimmediately suspended, and the alarm light or buzzer is activated. Thepredetermined fourth value is set to be larger than the value of theelectric-current flowing through the cutting tool 348 and the workpiece32 when the two members are shorted to each other by the cutting fluid.In either of this actual cutting stage and the above-described initialstage of the operation, the operator can take check the cutting tool 348or cutting insert 412 in response to the activation of the alarm lightor buzzer, so as to take a necessary procedure.

[0228] As is clear from the above description, the portion of theconductive coating 413 which portion is interposed between the cuttinginsert 412 and the tool body 410 functions as a resistive coating, whilethe portion of the conductive coating which portion is interposedbetween the cutting insert 412 and the mater workpiece 92 upon thecontact of the two members 92, 412 functions as a conductive coating.Therefore, the value of the resistance R3 provided by the former portionis preferably large, while the value of the resistance R4 provided bythe latter portion is preferably small. In the present embodiment, theconductive coating 413 is formed in its entirety of a single materialand has a constant thickness over its entirety, for facilitatingmanufacturing of the cutting insert 412 coated with the conductivecoating 413. As described above, since the cross sectional area of theabove-described former portion of the conductive coating 413 isconsiderably larger than that of the above-described latter portion ofthe conductive coating 413, the value of the resistance R3 is madeconsiderably smaller than that of the resistance R4. However, it isdesirable that value of the resistance R3 is larger than that of theresistance R4, in view of the above-described functions of the twoportions of the conductive coating 413. Therefore, it is preferable thatat least one of the values of the resistances R3, R4 is changed bymaking one of the two portions of the coating 413 different from theother portion in its thickness and/or material, such that at least oneof an increase in the value of the resistance R3 and a reduction in thevalue of the resistance R4 is made. In this sense, the conductivecoating 413 may be separated into two portions 413 a, 413 b, as shown inFIG. 15. In this modified arrangement, the portion 413 a is interposedbetween the cutting insert 412 and the tool body 410, while the portion413 b is interposed between the cutting insert 412 and the masterworkpiece 92. The portions 413 a, 413 b are different in thicknessand/or material from each other.

[0229] Further, as is apparent from the above descriptions, a portion ofthe computer 80, which portion is assigned to detect the contactposition in which the cutting point of the cutting tool 348 is broughtinto contact with the master workpiece 92, constitutes a control deviceas defined in mode (19) which is described above in SUMMARY OF THEINVENTION. The first circuit 96 and the current detector 100 constitutea contact detecting device as defined in mode (18). A portion of thecomputer 80 which portion is assigned to execute the above-describedchecking step cooperates with the second circuit 98 to constitute achecking device as defined in mode (18). A portion of the computer 80which portion is assigned to execute the above-described contactdetermining step constitutes a contact determining device as defined inmode (18). The first circuit 96 constitutes an on-off circuit as definedmode (35).

[0230] While the fixed resistor 106 in which the terminals 116, 118 aredisposed in the opposite ends of the resistive body 114 is used in thisembodiment, the fixed resistor 106 may be replaced by a resistive layersimilar to the resistive layer 170 in the second embodiment of FIG. 5,such that the resistive layer is interposed between the cutting tool 348and the turret 46.

[0231] While the cutting tool 348 is brought into contact at its cuttingpoint with the outer circumferential surface of the master workpiece 92in this embodiment, the cutting tool 348 may be brought into contactwith an outer circumferential surface of the workpiece 32 as in thethird embodiment of FIG. 6, or may be brought into contact with an outercircumferential surface of the main body 178 of the chuck 28 as in thefourth embodiment of FIG. 7. In the latter case, the main body 178 ofthe chuck 28 corresponds to a reference portion as defined in mode (8)which is described above in SUMMARY OF THE INVENTION. Where the cuttingblade is brought into contact with the reference portion, it isnecessary to surely prevent the reference portion from being damaged bythe cutting blade. The damage of the reference portion can beeffectively avoided, by bringing the cutting blade and the referenceportion into contact with each other via the conductive coating which isprovided to cover at least the cutting edge portion of the cuttingblade, namely by using the cutting blade which is coated at its surfacewith the conductive coating. The use of the cutting blade coated withthe coating is also effective to prevent chipping of the cutting blade,particularly, where the reference portion is hardened for preventing theportion from being damaged.

[0232] It is also possible to cover a surface of the cutting blade, thecutting tool or the cutting-tool holding member, with an electricallyinsulating coating. FIG. 16 shows a eleventh embodiment of the inventionin which the tool body 410 of the cutting tool 348 is covered at itssurface with an electrically insulating film or coating 481. Theprovision of the insulating coating 481 on the surface of the tool body410 is effective to prevent the turret 46 and the tool body 410 of thecutting tool 348 from being shorted at their mutually adjacent portionsto each other by a cutting fluid, cutting chips or other substancesticking to the surfaces of the turret 46 and the tool body 410 of thecutting tool 348, namely, prevent electrical connection between themutually adjacent portions through a by-passing passage which is formedof the sticking substance and which is positioned in parallel with thefixed resistor 106 of the second circuit 98. Therefore, this arrangementadvantageously avoids an erroneous determination that the cutting insert412 of the cutting tool 348 has been brought into contact with theworkpiece 32 or master workpiece 92. Such an erroneous determinationcould be caused by an increase in the detected value of the electriccurrent as a result of the electrical connection between the turret 46and the cutting tool 348, which is effectively prevented by theinsulating coating 481 in this eleventh embodiment. The above-describedcleaning step may be substituted for the provision of this electricallyinsulating coating 481 on the surface of the cutting tool, oralternatively may be implemented together with the provision of thisinsulating coating 481. In the latter case, the above-describederroneous determination can be more surely avoided. It is noted that theelectrically insulating coating 481 is formed on the surface of the toolbody 410 of the cutting tool 348 such that the insulating coating 181covers the surface of the tool body 410 except portions of the surfacewhich are to be held in contact with the cutting blade 412, the terminal116 of the resistor 106 and the contact member 162.

[0233] It is also possible to form a conductive coating on a surface ofa rotary cutting tool. FIG. 17 shows, as an example of the rotarycutting tool 192, an end mill 502 constructed according to a twelfthembodiment of the invention. The end mill 502 consists of a cuttingblade portion 503, a shank portion 505 and a conductive coating 504which covers the entire surface of the cutting blade portion 503. Theconductive coating 504 may be adapted to cover not only the cuttingblade portion 503 but also the shank portion 505. However, in thisembodiment, the shank portion 205 is not covered with the conductivecoating 504. The cutting blade portion 503 does not have to be coveredat its entire surface with the conductive coating 504, but may becovered at least a cutting edge and its adjacent portion with thecoating 540. However, in this embodiment, the cutting blade portion 503is covered at its entire surface with the coating 504, for facilitatingthe formation of the coating 504.

[0234]FIG. 18 shows, as another example of the rotary cutting tool 192,an end mill 554 constructed according to a thirteenth embodiment of theinvention. This end mill 554 includes a cutting blade in the form of areplaceable cutting insert 554 which is replaceably fixed to a main bodyof the end mill 554. The cutting inset 554 is covered with a conductivecoating.

[0235] Referring next to FIG. 19, there will be described acutting-blade-position controlling apparatus which is constructedaccording to a fourteenth embodiment of this invention. Thiscutting-blade-position controlling apparatus is built in a machiningcenter, a milling machine or a boring machine. An operation for millingor cutting the workpiece 193 with the rotary cutting tool 192 which iscovered with the conductive coating 504, is carried-out as follows:

[0236] The operation is initiated by attaching the tool holder 194 whichholds the rotary cutting tool 192, and the workpiece 193, to the toolspindle 196 and a jig (or fixture) 506, respectively, so that first andsecond circuits 507, 508 are formed. Described specifically, the DCpower supply 104, the current detector 100, the tool holder 194, therotary cutting tool 192, the reference portion 184 (or the workpiece193, the jig 506) and the main structure 182 are arranged in series inthe first circuit 507, while the DC power source 104, the currentdetector 100, the tool holder 194, the resistive layer 198, the toolspindle 196 and the main structure 182 are arranged in series in thesecond circuit 508.

[0237] After the formation of the first and second circuits 507, 508, achecking step is implemented by checking the electric current flowingthrough the second circuit 208, while a contact-position determiningstep is implemented by moving at least one of the tool spindle 196 andthe reference portion 184 toward each other. During this movement towardeach other, the cutting tool 192 is rotated at a velocity substantiallyequal to that at which the cutting tool 192 is rotated in a cuttingoperation for cutting the workpiece 193. When the cutting tool 192 isbrought into contact with one of the reference surfaces 186, 188, 190 ofthe reference portion 184, the first circuit 507 is closed for allowingthe electric current to flow through the first circuit 507 as well asthe second circuit 508. The increase in the detected value of theflowing electric current enables the computer 80 to detect thetransition of the first circuit 507 from its open state to its closedstate. A relative position of the cutting tool 192 and the referenceportion 184 upon the detection of the transition of the first circuit507 is detected as an indirect contact position. A direct contactposition of the cutting tool 192 and the reference portion 184 can beobtained, by offsetting the indirect contact position by an amountcorresponding to the thickness of the conductive coating 504.

[0238] In this embodiment in which the cutting tool 192 is being rotatedupon its contact with the reference portion 184, even where the cuttingtool 192 has a plurality of cutting teeth or edges whose respectiveradial distances from the rotary axis of the cutting tool 192 aredifferent from each other, it is possible to determine, as the contactposition, the relative position in which the reference portion 184 isbrought into contact with one of the cutting edges which has a largerradial distance from the rotary axis than the other cutting edges. Sincethe cutting tool 192 is brought into contact with the reference portion184 via the conductive coating 504, there is no risk of cut of thereference portion 184 in spite of the rotation of the cutting tool 192upon its contact with the reference portion 184.

[0239] Data representative of the contact position are stored, ascontact position data, in a contact-position memory of the RAM 84 of thecomputer 80. If there are previous contact-position data with respect tothe same cutting tool 192 in the contact-position memory, the previouscontact-position data are replaced with the new contact-position data.The workpiece 193 is cut by the cutting tool 192 on the basis of the newor renovated contact-position data. The cutting tool 192 is firstbrought into contact with the workpiece 193 via the conductive coating504, and then brought into direct contact with the workpiece 193 afterdestruction of the conductive coating 504, so as to start cutting theworkpiece 193.

[0240] The diamond coating 504 covering the main body 503 of the rotarycutting tool 192 may be formed of a material, which is selected fromamong a plurality of materials having different electric resistancevalues. For example, where five kinds of cutting tools 192 whoserespective axial lengths and/or diameters are different from each otherare used for cutting the workpiece 193 at a machining center having anATC (automatically tool changing) device, the main bodies 503 of thecutting tools 192 may be covered with the conductive coatings 504 whichare made of five kinds of materials having respective electricresistance values different from each other and which have respectivethicknesses equal to each other. Each of the main bodies 503 of thecutting tools 192 may be covered with one of the conductive coatings504, which one is selected depending upon the kind of the cutting tool192. In this arrangement, the value of the electric current flowingthrough the first circuit 507 upon the contact of each cutting tool 192with the workpiece 193 varies depending upon the kind of the cuttingtool 192. Described more specifically, when each cutting tool 192 isbrought into contact with the workpiece 193, the value of the electriccurrent flowing through the first and second circuits 504, 508 isdetected so that the detected value is compared with five values whichhave been predetermined for the respective five kinds of cutting tools192. It is then determined which one of the predetermined five values isclosest to the detected value, or which one of the predetermined fivevalues is substantially equal to the detected value with a differencetherebetween being not larger than a predetermined amount. It isaccordingly possible to know which one of the five kinds corresponds tothe cutting tool 192 currently brought into contact with the workpiece193.

[0241] Where it is known which one of the cutting tools 192 should becurrently brought into contact with the workpiece 193, the detectedvalue is compared with the predetermined value corresponding to thecutting tool 192 which should be currently brought into contact with theworkpiece 193. If the detected value is substantially equal to thepredetermined value with a difference therebetween being not larger thana predetermined amount, it can be determined that the cutting tool 192currently fixed to the tool spindle 196 is a currently desired cuttingtool, i.e., the cutting tool which should be currently brought intocontact with the workpiece 193. If the difference between the detectedvalue and the predetermined value is larger than the predeterminedamount, it can be determined that the cutting tool 192 currently fixedto the tool spindle 196 is not the currently desired cutting tool, andthat the cutting tool 192 is erroneously attached as a wrong cuttingtool to the tool spindle 196. Therefore, this arrangement is effectiveto avoid a dangerous situation in which the operation is proceeded withthe wrong cutting tool due to an erroneous operation of the operation ora malfunction of the ATC device of the machining center. It is notedthat the conductive coating 204 may be considered as a kind of resistivecoating in this embodiment, as is clear from the above description.

[0242] The conductive coating (resistive coating) does not have to covernecessarily the entire surface of the cutting blade portion of therotary cutting tool, but may cover only a cutting edge and its adjacentportion, or only a further limited portion (e.g., an axially distal endportion of a drill). Further, the conductive coating may be adapted tocover the surface of a shank portion of the cutting tool at whichportion the cutting tool is attached to the tool holding member. In anyone of these arrangements, it is possible to identify the cutting tooland check if the currently attached cutting tool is a desired cuttingtool or not. Further, also in a cutting tool including a main body and areplaceable cutting insert which is replaceably attached to the mainbody, the cutting inert may be covered (at its entirety, the cuttingedge and its adjacent portion, or the contact portion to be held incontact with the main body) with a conductive coating made of amaterial, which is selected among a plurality of materials havingrespective electric resistance values different from each other, so thatthe cutting insert attached to the main body of the tool can beidentified in substantially the same manner as described above. That is,the present technique for identifying the cutting tool can be applied toall types of cutting tools to be used in operations with machine tools.

[0243] While the cutting blade and the mater workpiece are made ofmaterials each having a high degree of electrical conductivity in theabove-described embodiments, these members may be made of electricallyinsulating materials or resistive materials. FIG. 20 shows a fifteenthembodiment of the invention in which a replaceable cutting insert 528 ismade of a ceramic material or other electrically insulating materialhaving a high degree of electric resistance. The cutting insert 528 iscovered at its entire surface with an electrically conductive coating529. This cutting insert 528 covered with the conductive coating 529 canbe used in the same manner as the above-described cutting inserts madeof the conductive materials, until a portion of the conductive coating529 which portion covers a cutting edge and its adjacent potion isdestroyed or removed as a result of a long service for cuttingoperations.

[0244] While the cutting blade is covered with the conductive coating,the object may be covered, at a portion of its surface which is broughtinto contact with the cutting blade, with the conductive surface. FIG.21 shows a sixteenth embodiment of the invention in which a conductivecoating 532 is provided to cover a surface of a master workpiece 530which has a known dimension or dimensions and which is to be attached toa workpiece holding member serving to hold a workpiece to be cut. Thismaster workpiece 530 can be advantageously used with a standard cuttingblade which is not covered with a conductive coating, for avoiding riskof chipping of cutting blade or damage of the master workpiece when themaster workpiece 530 and the cutting blade are brought into contact witheach other.

[0245]FIG. 22 shows a sixteenth embodiment of the invention in which anelectrically conductive sheet 544 is positioned to be interposed betweena cutting blade 540 and an object 542 when the cutting blade 540 and theobject 542 are in contact with each other. Also in this arrangement, itis possible to avoid chipping of the cutting blade 540 or damage of theobject 542, owing to the interposition of the conductive sheet 544between the two members 540, 542.

[0246] It is to be understood that the present invention may be embodiedwith various other changes, modifications and improvements, such asthose described in the SUMMARY OF THE INVENTION, which may occur tothose skilled in the art, without departing from the spirit and scope ofthe invention defined in the following claims:

What is claimed:
 1. A method of controlling a relative movement of acutting blade and a workpiece which are moved relative to each other bya movement device in an operation with a machine tool, said methodcomprising a step of bringing said cutting blade and an object intocontact with each other, by moving at least one of said cutting bladeand said object toward each other by said movement device, and a step ofcontrolling said relative movement on the basis of a relative positionof said cutting blade and said object which is detected by wherein saidmovement device upon said contact of said cutting blade and said objectwith each other, said object consists of said workpiece which is fixedto said machine tool, said method further comprising: a dimensionmeasuring step of measuring a dimension of said workpiece on the basisof said relative position of said cutting blade and said workpiece assaid object upon said contact of said cutting blade and said workpieceas said object with each other, so that said relative movement iscontrollable on the basis of the measured dimension of said workpiece.2. An apparatus for controlling a relative movement of a cutting bladeand a workpiece which are moved relative to each other by a movementdevice in an operation with a machine tool, said apparatus controllingsaid relative movement on the basis of a relative position of saidcutting blade and an object which is detected by said movement devicewhen said cutting blade and said object are brought into contact witheach other as a result of a relative movement of said cutting blade andsaid object which is made by said movement device, said apparatuscomprising: a checking device which checks if a contact detecting devicefor detecting contact of said cutting blade and said object is in anormal condition in which said contact detecting device detects saidcontact when said cutting blade and said object are brought into contactwith each other; and a contact determining device which determines thatsaid cutting blade and said object have been brought into contact witheach other, in accordance with an output provided by said contactdetecting device
 3. A method of detecting contact and separation of acutting blade held by a blade holding member, with and from an object,on the basis of a change of a state of an electric circuit which ischanged depending upon whether said cutting blade is in contact withsaid object or is separated from said object, said method comprising: astep of bringing said cutting blade and said object into contact witheach other, while a conductive layer having an electrical conductivityis provided in at least one of a space between said cutting blade andsaid blade holding member, and a space between said cutting blade andsaid object.
 4. A method according to claim 3, wherein said contact andsaid separation are detected on the basis of transition from an openstate in which an on-off circuit as said electric circuit is open, to aclosed state in which said on-off circuit is closed, and wherein saidon-off circuit includes at least said cutting blade, said object and apower source which are arranged in series to each other, said on-offcircuit being open when said cutting blade is separated from said objectwhile being closed when said cutting blade is in contact with saidobject.
 5. A method according to claim 3, wherein said contact and saidseparation are detected on the basis of transition from an open state inwhich an on-off circuit is open, to a closed state in which said on-offcircuit is closed, and wherein said on-off circuit includes at leastsaid blade holding member, said cutting blade, said object and a powersource which are arranged in series to each other, said on-off circuitbeing open when said cutting blade is separated from said object whilebeing closed when said cutting blade is in contact with said object. 6.A method according to claim 3, wherein said conductive layer consists ofa conductive coating which covers a surface of said cutting blade.
 7. Amethod according to claim 4, wherein said conductive layer consists of aconductive coating which covers a contact surface of said object whichsurface is in contact with said cutting blade.
 8. A method according toclaim 7, wherein said object comprises a master workpiece which has aknown dimension and which is held by a workpiece holding device that isprovided for holding a workpiece to be cut by said cutting blade.
 9. Amethod according to claim 4, wherein said conductive layer consists of aconductive sheet which is positioned to be interposed between saidcutting blade and said object when said cutting blade and said objectare in contact with each other.
 10. A method according to claim 3,wherein said cutting blade is provided by at least a cutting edge of arotary cutting tool which is to be rotated about an axis thereof forcutting a workpiece, and an adjacent portion of said rotary cutting toolwhich portion is adjacent to said cutting edge, wherein said cuttingedge and said adjacent portion is covered with a conductive coating assaid conductive layer, and said rotary cutting tool is brought intocontact with said object while said rotary cutting tool is beingrotated.
 11. A method according to claim 10, wherein said rotary cuttingtool is brought into contact with said object while said rotary cuttingtool is being rotated at a velocity substantially equal to that at whichsaid rotary cutting tool is rotated in a cutting operation for cuttingsaid workpiece.
 12. A cutting blade which is removably held by a bladeholding member, for thereby cutting a workpiece, said cutting bladebeing covered at at least a portion of a surface thereof with aconductive coating having an electrical conductivity.
 13. A cuttingblade according to claim 12, wherein said conductive coating is made ofa material whose electric resistance is larger than that of a materialof said cutting blade.
 14. A cutting blade according to claim 12,wherein said conductive coating is made of a material whose electricresistance is smaller than that of a material of said cutting blade. 15.A cutting blade according to claim 12, being covered at all the surfacewith said conductive coating.
 16. A cutting blade according to claim 12,comprising a replaceable cutting insert which is replaceably attached toa main body of a cutting tool, said main body including a shank portionand cooperating with said replaceable cutting insert to constitute saidcutting tool.
 17. A cutting blade according to claim 12, comprising acutting edge of a rotary cutting tool, and an adjacent portion of saidrotary cutting tool which portion is adjacent to said cutting edge, saidrotary cutting tool being rotated about an axis thereof for achieving acutting operation.
 18. A cutting blade according to claim 17, whereinsaid cutting edge and said adjacent portion are provided by areplaceable cutting insert which is replaceably attached to a main bodyof said rotary cutting tool, said main body including a shank portionand cooperating with said replaceable cutting insert to constitute saidrotary cutting tool.
 19. A master workpiece which is to be held by aworkpiece holding device serving for holding a workpiece to be cut by acutting blade, and which is to be brought into contact with said cuttingblade, said master workpiece having a known dimension and covered at asurface thereof with a conductive coating which has an electricalconductivity.
 20. A conductive sheet consisting of a sheet member whichis made a material having an electrical conductivity.